U.S. patent application number 16/909299 was filed with the patent office on 2020-10-08 for laser-assisted method for parting crystalline material.
The applicant listed for this patent is Cree, Inc.. Invention is credited to Matthew Donofrio, John Edmond, Harshad Golakia.
Application Number | 20200316724 16/909299 |
Document ID | / |
Family ID | 1000004915386 |
Filed Date | 2020-10-08 |
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United States Patent
Application |
20200316724 |
Kind Code |
A1 |
Donofrio; Matthew ; et
al. |
October 8, 2020 |
LASER-ASSISTED METHOD FOR PARTING CRYSTALLINE MATERIAL
Abstract
A method for processing a crystalline substrate to form multiple
patterns of subsurface laser damage facilitates subsequent fracture
of the substrate to yield first and second substrate portions of
reduced thickness. Multiple (e.g., two, three, or more) groups of
parallel lines of multiple subsurface laser damage patterns may be
sequentially interspersed with one another, with at least some
lines of different groups not crossing one another. Certain
implementations include formation of multiple subsurface laser
damage patterns including groups of parallel lines that are
non-parallel to one another, but with each line remaining within
.+-.5 degrees of perpendicular to the <1120> direction of a
hexagonal crystal structure of a material of the substrate. Further
methods involve formation of initial and subsequent subsurface
laser damage patterns that are centered at different depths within
an interior of a substrate, with the subsurface laser damage
patterns being registered with one another and having vertical
extents that are overlapping.
Inventors: |
Donofrio; Matthew; (Raleigh,
NC) ; Edmond; John; (Durham, NC) ; Golakia;
Harshad; (Morrisville, NC) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Cree, Inc. |
Durham |
NC |
US |
|
|
Family ID: |
1000004915386 |
Appl. No.: |
16/909299 |
Filed: |
June 23, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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PCT/IB2019/061410 |
Dec 27, 2019 |
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16909299 |
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16274064 |
Feb 12, 2019 |
10576585 |
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PCT/IB2019/061410 |
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62803340 |
Feb 8, 2019 |
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62786333 |
Dec 29, 2018 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B23K 2103/56 20180801;
B23K 2101/40 20180801; H01L 21/02021 20130101; H01L 21/67092
20130101; H01L 21/67115 20130101; H01L 21/6835 20130101; H01L
21/6838 20130101; B23K 26/0876 20130101; H01L 21/30625 20130101;
B23K 26/0853 20130101; B23K 26/359 20151001; H01L 21/02013
20130101; H01L 21/8213 20130101; H01L 21/268 20130101; B23K 26/0006
20130101; B23K 26/53 20151001; H01L 29/1608 20130101; B23K 26/0665
20130101 |
International
Class: |
B23K 26/53 20060101
B23K026/53; B23K 26/06 20060101 B23K026/06; B23K 26/08 20060101
B23K026/08; B23K 26/359 20060101 B23K026/359; H01L 21/02 20060101
H01L021/02; H01L 21/268 20060101 H01L021/268; H01L 21/306 20060101
H01L021/306; H01L 21/67 20060101 H01L021/67; H01L 21/683 20060101
H01L021/683; H01L 21/82 20060101 H01L021/82; H01L 29/16 20060101
H01L029/16; B23K 26/00 20060101 B23K026/00 |
Claims
1. A crystalline material processing method comprising: splitting
emissions of a single laser to form first and second split laser
emissions; supplying the first split laser emissions focused within
an interior of a first portion of a crystalline material substrate
and supplying the second split laser emissions focused within an
interior of a second portion of the crystalline material substrate,
while effecting relative lateral movement between (i) the laser and
(ii) the crystalline material substrate, to form initial subsurface
laser damage in each of the first and second portions of the
crystalline material substrate simultaneously, the initial
subsurface laser damage having an initial subsurface laser damage
pattern comprising an initial plurality of substantially parallel
lines; and supplying the first split laser emissions focused within
the interior of the first portion of the crystalline material
substrate and supplying the second split laser emissions focused
within the interior of the second portion of the crystalline
material substrate, while effecting relative lateral movement
between (i) the laser and (ii) the crystalline material substrate,
to form subsequent subsurface laser damage in each of the first and
second portions of the crystalline material substrate
simultaneously, the subsequent subsurface laser damage having a
subsequent subsurface laser damage pattern comprising a subsequent
plurality of substantially parallel lines; wherein, within each of
the first and second portions of the crystalline material
substrate, at least some lines of the subsequent plurality of
substantially parallel lines do not cross any lines of the initial
plurality of substantially parallel lines.
2. The crystalline material processing method of claim 1, wherein,
within each of the first and second portions of the crystalline
material substrate, lines of the initial plurality of substantially
parallel lines are substantially parallel to lines of the
subsequent plurality of substantially parallel lines.
3. The crystalline material processing method of claim 1, wherein,
within each of the first and second portions of the crystalline
material substrate, lines of the initial plurality of substantially
parallel lines are non-parallel to lines of the subsequent
plurality of substantially parallel lines, and wherein an angular
direction of lines of the subsequent plurality of substantially
parallel lines differs by no more than 10 degrees from an angular
direction of lines of the initial plurality of substantially
parallel lines.
4. The crystalline material processing method of claim 1, wherein,
within each of the first and second portions of the crystalline
material substrate, each line of the subsequent plurality of
substantially parallel lines does not cross any lines of the
initial plurality of substantially parallel lines.
5. The crystalline material processing method of claim 1, wherein,
within each of the first and second portions of the crystalline
material substrate, lines of the subsequent plurality of
substantially parallel lines are interspersed among lines of the
initial plurality of substantially parallel lines.
6. The crystalline material processing method of claim 1, wherein:
within each of the first and second portions of the crystalline
material substrate, the initial subsurface laser damage pattern
comprises a first subsurface laser damage pattern including a first
plurality of substantially parallel lines, and a second subsurface
laser damage pattern including a second plurality of substantially
parallel lines; within each of the first and second portions of the
crystalline material substrate, the subsequent laser damage pattern
comprises a third subsurface laser damage pattern; and within each
of the first and second portions of the crystalline material
substrate, lines of the third plurality of substantially parallel
lines are interspersed among lines of the first plurality of
substantially parallel lines and the second plurality of
substantially parallel lines, with each line of the third plurality
of substantially parallel lines being arranged between one line of
the first plurality of substantially parallel lines and one line of
the second plurality of substantially parallel lines.
7. The crystalline material processing method of claim 6, wherein,
within each of the first and second portions of the crystalline
material substrate, each line of the first plurality of
substantially parallel lines is separated from a nearest line of
the second plurality of substantially parallel lines by at least
100 microns.
8. The crystalline material processing method of claim 6, wherein:
within each of the first and second portions of the crystalline
material substrate, the first subsurface laser damage pattern
comprises a first plurality of cracks in the interior of the
crystalline material substrate propagating laterally outward from
lines of the first plurality of substantially parallel lines;
within each of the first and second portions of the crystalline
material substrate, the second subsurface laser damage pattern
comprises a second plurality of cracks in the interior of the
crystalline material substrate propagating laterally outward from
lines of the second plurality of substantially parallel lines, and
the second plurality of cracks is non-connecting with the first
plurality of cracks; and within each of the first and second
portions of the crystalline material substrate, the third
subsurface laser damage pattern comprises a third plurality of
cracks in the interior of the crystalline material substrate
propagating laterally outward from lines of the third plurality of
substantially parallel lines, wherein at least some cracks of the
third plurality of cracks connect with at least some cracks of the
first plurality of cracks and with at least some cracks of the
second plurality of cracks.
9. The crystalline material processing method of claim 1, wherein:
the crystalline material substrate comprises a hexagonal crystal
structure; and for each of the first and second portions of the
crystalline material substrate, each line of the initial plurality
of substantially parallel lines and each line of the subsequent
plurality of substantially parallel lines is within .+-.5 degrees
of perpendicular to a <1120> direction of the hexagonal
crystal structure and substantially parallel to a flat surface of
the crystalline material substrate.
10. The crystalline material processing method of claim 1, further
comprising, for each of the first and second portions of the
crystalline material substrate, fracturing the crystalline material
substrate substantially along at least one of, or between, the
initial subsurface laser damage pattern and the subsequent
subsurface laser damage pattern, to yield first and second
crystalline material portions of the crystalline material substrate
each having reduced thickness relative to the crystalline material
substrate, but substantially a same length and width as the
crystalline material substrate.
11. The crystalline material processing method of claim 10, wherein
at least one of the first crystalline material portion or the
second crystalline material portion comprises a free-standing wafer
configured for growth of at least one epitaxial layer thereon.
12. The crystalline material processing method of claim 10, wherein
one of the first crystalline material portion or the second
crystalline material portion comprises a device wafer including at
least one epitaxial layer grown thereon.
13. A crystalline material processing method comprising: splitting
emissions of a single laser to form first and second split laser
emissions; supplying the first split laser emissions focused within
an interior of a first substrate of crystalline material and
supplying the second split laser emissions focused within an
interior of a second substrate of crystalline material, and
effecting relative lateral movement between (i) the laser and (ii)
the first and second substrates, to form initial subsurface laser
damage in each of the first and second substrates simultaneously,
the subsurface laser damage having an initial subsurface laser
damage pattern comprising an initial plurality of substantially
parallel lines; and supplying the first split laser emissions
focused within the interior of the first substrate and supplying
the second split laser emissions focused within the interior of the
second substrate, and effecting relative lateral movement between
the (i) laser and (ii) the first and second substrates, to form
subsequent subsurface laser damage in each of the first and second
substrates simultaneously, the subsequent subsurface laser damage
having a subsequent subsurface laser damage pattern comprising a
subsequent plurality of substantially parallel lines; wherein,
within each of the first and second substrates, at least some lines
of the subsequent plurality of substantially parallel lines do not
cross any lines of the initial plurality of substantially parallel
lines.
14. The crystalline material processing method of claim 13,
wherein, within each of the first and second substrates, each line
of the subsequent plurality of substantially parallel lines does
not cross any lines of the initial plurality of substantially
parallel lines.
15. The crystalline material processing method of claim 13,
wherein, within each of the first and second substrates, lines of
the subsequent plurality of substantially parallel lines are
interspersed among lines of the initial plurality of substantially
parallel lines.
16. The crystalline material processing method of claim 13,
wherein: within each of the first and second substrates, the
initial subsurface laser damage pattern comprises a first
subsurface laser damage pattern including a first plurality of
substantially parallel lines, and a second subsurface laser damage
pattern including a second plurality of substantially parallel
lines; within each of the first and second substrates, the
subsequent laser damage pattern comprises a third subsurface laser
damage pattern; and within each of the first and second substrates,
lines of the third plurality of substantially parallel lines are
interspersed among lines of the first plurality of substantially
parallel lines and the second plurality of substantially parallel
lines, with each line of the third plurality of substantially
parallel lines being arranged between one line of the first
plurality of substantially parallel lines and one line of the
second plurality of substantially parallel lines.
17. The crystalline material processing method of claim 16,
wherein, within each of the first and second substrates, each line
of the first plurality of substantially parallel lines is separated
from a nearest line of the second plurality of substantially
parallel lines by at least 100 microns.
18. The crystalline material processing method of claim 16,
wherein: within each of the first and second substrates, the first
subsurface laser damage pattern comprises a first plurality of
cracks in the interior of the crystalline material propagating
laterally outward from lines of the first plurality of
substantially parallel lines; within each of the first and second
substrates, the second subsurface laser damage pattern comprises a
second plurality of cracks in the interior of the crystalline
material propagating laterally outward from lines of the second
plurality of substantially parallel lines, and the second plurality
of cracks is non-connecting with the first plurality of cracks; and
within each of the first and second substrates, the third
subsurface laser damage pattern comprises a third plurality of
cracks in the interior of the crystalline material propagating
laterally outward from lines of the third plurality of
substantially parallel lines, wherein at least some cracks of the
third plurality of cracks connect with at least some cracks of the
first plurality of cracks and with at least some cracks of the
second plurality of cracks.
19. A crystalline material processing method comprising: mounting
first and second substrates of crystalline material to a holder,
the first substrate comprising a first plurality of areas that are
non-overlapping relative to one another, and the second substrate
comprising a second plurality of areas that are non-overlapping
relative to one another; splitting emissions of a single laser to
form first and second split laser emissions; supplying the first
split laser emissions to the first substrate and supplying the
second split laser emissions to the second substrate, to form a
first plurality of subsurface laser damage regions in each area of
the first plurality of areas and of the second plurality of areas;
supplying the first split laser emissions to the first substrate
and supplying the second split laser emissions to the second
substrate, to form a second plurality of subsurface laser damage
regions in each area of the first plurality of areas and of the
second plurality of areas, wherein within each of the first and
second substrates, at least some subsurface laser damage regions of
the first plurality of subsurface laser damage regions do not cross
subsurface laser damage regions of the second plurality of
subsurface laser damage regions.
20. The crystalline material processing method of claim 19, wherein
within each of the first and second substrates, the first plurality
of subsurface laser damage regions comprises a first plurality of
substantially parallel lines, and the second plurality of
subsurface laser damage regions comprises a second plurality of
substantially parallel lines.
21. The crystalline material processing method of claim 20,
wherein, within each of the first and second substrates, each line
of the first plurality of substantially parallel lines is separated
from a nearest line of the second plurality of substantially
parallel lines by at least 100 microns.
22. The crystalline material processing method of claim 19, wherein
within each of the first and second substrates, wherein each line
of the second plurality of substantially parallel lines does not
cross any lines of the first plurality of substantially parallel
lines.
23. The crystalline material processing method of claim 19, further
comprising: following formation of the first plurality of
subsurface laser damage regions and the second subsurface laser
damage regions, forming a third plurality of subsurface laser
damage regions (i) in each area of the first plurality of areas,
and (ii) in each area of the second plurality of areas, wherein:
for each of the first and second substrates, the first plurality of
subsurface laser damage regions comprises a first plurality of
substantially parallel lines; for each of the first and second
substrates, the second plurality of subsurface laser damage regions
comprises a second plurality of substantially parallel lines; for
each of the first and second substrates, the third plurality of
subsurface laser damage regions comprises a third plurality of
substantially parallel lines; and for each of the first and second
substrates, at least some lines of the third plurality of
substantially parallel lines are interspersed among lines of the
first plurality of substantially parallel lines and the second
plurality of substantially parallel lines
Description
STATEMENT OF RELATED APPLICATIONS
[0001] This application is a continuation of International
Application No. PCT/IB2019/061410 filed on Dec. 27, 2019, which
claims priority to U.S. patent application Ser. No. 16/274,064
filed on Feb. 12, 2019 and subsequently issued as U.S. Pat. No.
10/576,585 on Mar. 3, 2020, claims priority to U.S. Provisional
Patent Application No. 62/803,340 filed on Feb. 8, 2019, and claims
priority to U.S. Provisional Patent Application No. 62/786,333
filed on Dec. 29, 2018, wherein the entire disclosures of the
foregoing applications and patent are hereby incorporated by
reference herein.
TECHNICAL FIELD
[0002] The present disclosure relates to methods for processing
crystalline materials, and more specifically to laser-assisted
methods for parting or removing relatively thin layers of
crystalline material from a substrate, such as a boule or a
wafer.
BACKGROUND
[0003] Various microelectronic, optoelectronic, and
microfabrication applications require thin layers of crystalline
materials as a starting structure for fabricating various useful
systems. Traditional methods for cutting thin layers (e.g., wafers)
from large diameter crystalline ingots of crystalline materials
have involved use of wire saws. Wire sawing technology has been
applied to various crystalline materials, such as silicon,
sapphire, and silicon carbide. A wire saw tool includes an
ultra-fine steel wire (typically having a diameter of 0.2 mm or
less) that is passed through grooves of one or many guide rollers.
Two slicing methods exist, namely, loose abrasive slicing and fixed
abrasive slicing. Loose abrasive slicing involves application of a
slurry (typically a suspension of abrasives in oil) to a steel wire
running at high speed, whereby the rolling motion of abrasives
between the wire and the workpiece results in cutting of an ingot.
Unfortunately, the environmental impact of slurry is considerable.
To reduce such impact, a wire fixed with diamond abrasives may be
used in a fixed abrasive slicing method that requires only a
water-soluble coolant liquid (not a slurry). High-efficiency
parallel slicing permits a large number of wafers to be produced in
a single slicing procedure. FIG. 1 illustrates a conventional wire
saw tool 1 including parallel wire sections 3 extending between
rollers 4A-4C and arranged to simultaneously saw an ingot 2 into
multiple thin sections (e.g., wafers 8A-8G) each having a face
generally parallel to an end face 6 of the ingot 2. During the
sawing process, the wire sections 3 supported by the rollers 4A-4C
may be pressed in a downward direction 5 toward a holder 7
underlying the ingot 2. If the end face 6 is parallel to a
crystallographic c-plane of the ingot 2, and the wire sections 3
saw through the ingot 2 parallel to the end face 6, then each
resulting wafer 8A-8G will have an "on-axis" end face 6' that is
parallel to the crystallographic c-plane.
[0004] It is also possible to produce vicinal (also known as offcut
or "off-axis") wafers having end faces that are not parallel to the
crystallographic c-plane. Vicinal wafers (e.g., of SiC) having a 4
degree offcut are frequently employed as growth substrates for
high-quality epitaxial growth of other materials (e.g., AlN and
other Group III nitrides). Vicinal wafers may be produced either by
growing an ingot in a direction away from the c-axis (e.g., growing
over a vicinal seed material) and sawing the ingot perpendicular to
the ingot sidewalls), or by growing an ingot starting with an
on-axis seed material and sawing the ingot at an angle to that
departs from perpendicular to the ingot sidewalls.
[0005] Wire sawing of semiconductor materials involves various
limitations. Kerf losses based on the width of material removed per
cut are inherent to saw cutting, and represent a significant loss
of semiconductor material. Wire saw cutting applies moderately high
stress to wafers, resulting in non-zero bow and warp
characteristics. Processing times for a single boule (or ingot) are
very long, and events like wire breaks can increase processing
times and lead to undesirable loss of material. Wafer strength may
be reduced by chipping and cracking on the cut surface of a wafer.
At the end of a wire sawing process, the resulting wafers must be
cleaned of debris.
[0006] In the case of silicon carbide (SiC) having high wear
resistance (and a hardness comparable to diamond and boron
nitride), wire sawing may require significant time and resources,
thereby entailing significant production costs. SiC substrates
enable fabrication of desirable power electronic, radio frequency,
and optoelectronic devices. SiC occurs in many different crystal
structures called polytypes, with certain polytypes (e.g., 4H-SiC
and 6H-SIC) having a hexagonal crystal structure.
[0007] FIG. 2 is a first perspective view crystal plane diagram
showing the coordinate system for a hexagonal crystal such as
4H-SiC, in which the c-plane ((0001) plane, corresponding to a
[0001] (vertical) direction of epitaxial crystal growth) is
perpendicular to both the m-plane ((1100) plane) and the a-plane
((1120) plane), with the (1100) plane being perpendicular to the
[1100] direction, and the (1120) plane being perpendicular to the
[1120] direction. FIG. 3 is a second perspective view crystal plane
diagram for a hexagonal crystal, illustrating a vicinal plane 9
that is non-parallel to the c-plane, wherein a vector 10 (which is
normal to the vicinal plane 9) is tilted away from the [0001]
direction by a tilt angle .beta., with the tilt angle .beta. being
inclined (slightly) toward the [1120] direction. FIG. 4A is a
perspective view wafer orientation diagram showing orientation of a
vicinal wafer 11A relative to the c-plane ((0001) plane), in which
a vector 10A (which is normal to the wafer face 9A) is tilted away
from the [0001] direction by a tilt angle .beta.. This tilt angle
.beta. is equal to an orthogonal tilt (or misorientation angle)
.beta. that spans between the (0001) plane and a projection 12A of
the wafer face 9A. FIG. 4B is a simplified cross-sectional view of
the vicinal wafer 11A superimposed over a portion of an ingot 14A
(e.g., an on-axis ingot having an end face 6A parallel to the
(0001) plane) from which the vicinal wafer 11A was defined. FIG. 4B
shows that the wafer face 9A of the vicinal wafer 11A is misaligned
relative to the (0001) plane by a tilt angle .beta..
[0008] FIG. 5 is a top plan view of an exemplary SiC wafer 25
including an upper face 26 (e.g., that is parallel to the (0001)
plane (c-plane), and perpendicular to the [0001] direction) and
laterally bounded by a generally round edge 27 (having a diameter
D) including a primary flat 28 (having a length L.sub.F) that is
perpendicular to the (1120) plane, and parallel to the [1120]
direction. A SiC wafer may include an outer surface that is
misaligned with (e.g., off-axis at an oblique angle relative to)
the c-plane.
[0009] Due to difficulties associated with making and processing
SiC, SiC device wafers have a high cost relative to wafers of
various other semiconductor materials. Typical kerf losses obtained
from wire sawing SiC may be approximately 250 microns or more per
wafer, which is quite significant considering that the wafers
resulting from a wire sawing process may be roughly 350 microns
thick and subsequently thinned (by grinding) to a final thickness
of approximately 100 to 180 microns depending on the end use. It
has been impractical to slice wafers thinner than about 350 microns
considering wire sawing and device fabrication issues.
[0010] To seek to address limitations associated with wire sawing,
alternative techniques for removing thin layers of semiconductor
materials from bulk crystals have been developed. One technique
involving removal of a layer of silicon carbide from a larger
crystal is described in Kim et al., "4H-SiC wafer slicing by using
femtosecond laser double pulses," Optical Materials Express 2450,
vol. 7, no. 7 (2017). Such technique involves formation of
laser-written tracks by impingement of laser pulses on silicon
carbide to induce subsurface damage, followed by adhesion of the
crystal to a locking jig and application of tensile force to
effectuate fracture along a subsurface damage zone. Use of the
laser to weaken specific areas in the material followed by fracture
between those areas reduces the laser scanning time.
[0011] Another separation technique involving formation of laser
subsurface damage is disclosed by U.S. Pat. No. 9,925,619 to Disco
Corporation. Laser subsurface damage lines are formed by movement
of a SiC ingot in a forward path, indexing the focal point of the
laser, then moving the ingot in backward path, indexing the focal
point of the laser, and so on. The formation of laser subsurface
damage produces internal cracks extending parallel to a c-plane
within an ingot, and ultrasonic vibration is applied to the ingot
to introduce fracture.
[0012] A similar separation technique involving formation of laser
subsurface damage is disclosed by U.S. Pat. No. 10,155,323 to Disco
Corporation. A pulsed laser beam is supplied to a SiC ingot to form
multiple continuous modified portions each having a 17 micron
diameter with an overlap rate of 80% in the feeding direction, and
the focal point of the laser is indexed, with the modified portion
forming step and indexing step being alternately performed to
produce a separation layer in which cracks adjacent to each other
in the indexing direction are connected. Thereafter, ultrasonic
vibration is applied to the ingot to introduce fracture.
[0013] Another technique for removing thin layers of semiconductor
materials from bulk crystals is disclosed in U.S. Patent
Application Publication No. 2018/0126484A1 to Siltectra GmbH. Laser
radiation is impinged on a solid state material to create a
detachment zone or multiple partial detachment zones, followed by
formation of a polymer receiving layer (e.g., PDMS) and cooling
(optionally combined with high-speed rotation) to induce mechanical
stresses that cause a thin layer of the solid state material to
separate from a remainder of the material along the detachment
zone(s).
[0014] Tools for forming laser subsurface damage in semiconductor
materials are known in the art and commercially available from
various providers, such as Disco Corporation (Tokyo, Japan). Such
tools permit laser emissions to be focused within an interior of a
crystalline substrate, and enable lateral movement of a laser
relative to the substrate. Typical laser damage patterns include
formation of parallel lines that are laterally spaced relative to
one another at a depth within a crystalline material substrate.
Parameters such as focusing depth, laser power, translation speed,
etc. may be adjusted to impart laser damage, but adjustment of
certain factors involves tradeoffs. Increasing laser power tends to
impart greater subsurface damage that may increase ease of
fracturing (e.g., by reducing the stress required to complete
fracturing), but greater subsurface damage increases surface
irregularities along surfaces exposed by fracturing, such that
additional processing may be required to render such surfaces
sufficiently smooth for subsequent processing (e.g., for
incorporation in electronic devices). Reducing lateral spacing
between subsurface laser damage lines may also increase ease of
fracturing, but a reduction in spacing between laser damage lines
increases the number of translational passes between a substrate
and a laser, thereby reducing tool throughput. Additionally,
results obtained by laser processing may vary within a substrate,
depending on lateral or radial position at a particular vertical
position, and/or depending on vertical position of a substrate face
relative to its original growth position as part of an ingot.
[0015] Accordingly, the art continues to seek improved
laser-assisted methods for parting or removing relatively thin
layers of crystalline (e.g., semiconductor) material from a
substrate to address issues associated with conventional
methods.
SUMMARY
[0016] The present disclosure relates in various aspects to methods
for processing a crystalline material substrate to form multiple
subsurface laser damage sites in areas of the crystalline material
to facilitate subsequent fracture of the substrate to yield first
and second crystalline material portions. Formation of subsurface
laser damage is distributed among multiple non-overlapping areas of
the crystalline material. For example, a first group of subsurface
laser damage sites may be formed in non-overlapping first and
second areas of the crystalline material. Thereafter, a second
group of subsurface laser damage sites may be formed within the
same non-overlapping first and second areas of the crystalline
material, with at least some (or all) sites of the second group of
subsurface laser damage sites not crossing sites of the first group
of subsurface laser damage sites is formed in the non-overlapping
areas. Additional groups of subsurface laser damage sites can be
distributed among the same non-overlapping first and second areas
of the crystalline material until the desired amount of subsurface
laser damage has been formed. It has been found that by
distributing the subsurface laser damage in this manner, the
spacing between adjacent subsurface laser damage sites can be
increased (and spacing between non-overlapping areas can be
increased, if such areas are spaced apart) and less subsurface
laser damage may be required to part the crystalline material,
thereby enabling increased laser tool throughput and reduced kerf
losses.
[0017] In certain embodiments, each group of subsurface laser
damage sites is in the form of multiple parallel lines, and each
set of parallel lines in the non-overlapping areas of the
crystalline material form subsurface laser damage patterns. In
certain implementations, the multiple (e.g., first and second,
first through third, etc.) pluralities of substantially parallel
lines of multiple (e.g., first and second, first through third,
etc.) subsurface laser damage patterns are interspersed. In certain
implementations, at least some lines of a second plurality of
substantially parallel lines do not cross any lines of a first
plurality of substantially parallel lines. Certain embodiments
involve formation of an initial subsurface laser damage pattern and
a subsequent subsurface laser damage pattern each comprising a
plurality of substantially parallel lines in a substrate of
crystalline material comprising a hexagonal crystal structure,
wherein each line is within .+-.5 degrees of perpendicular to a
<1120> direction of the hexagonal crystal structure, and
lines of the initial plurality of substantially parallel lines are
non-parallel to lines of the subsequent plurality of substantially
parallel lines. Further embodiments involve forming a first
plurality of subsurface laser damage regions in each area of a
plurality of areas of the crystalline material, and forming a
second plurality of subsurface laser damage regions in each area of
the plurality of areas of the crystalline material, wherein at
least some of regions of the first plurality of subsurface damage
regions do not cross regions of the second plurality of subsurface
damage regions. Additional embodiments involve sequentially forming
first and second pluralities of subsurface laser damage regions
across each area of the plurality of areas to form interspersed
subsurface laser damage regions, wherein at least some regions of
the first plurality of subsurface laser damage regions do not cross
regions of the second plurality of subsurface laser damage regions.
Still further embodiments involve formation of an initial
subsurface laser damage pattern substantially centered at an
initial depth within an interior of a crystalline material of a
substrate, and formation of a subsequent subsurface laser damage
pattern substantially centered at a subsequent depth within the
substrate, wherein the subsequent depth differs from the initial
depth, the subsequent subsurface laser damage pattern is
substantially registered with the initial subsurface laser damage
pattern, and vertical extents of at least portions of the initial
and subsurface laser damage patterns are overlapping. Each of the
foregoing methods may promote subsequent fracture of crystalline
material substrates. Additional methods involve processing
crystalline material with multiple grinding steps to remove
subsurface damage and edge grinding to impart a beveled or rounded
edge profile, wherein an order of grinding steps is selected and/or
a protective surface coating is employed to reduce the likelihood
of imparting additional surface damage after edge grinding and
render a wafer ready for chemical mechanical planarization.
Furthermore, a material processing apparatus includes a laser
processing station, a fracturing station, multiple coarse grinding
stations arranged in parallel downstream of the fracturing station,
and at least one fine grinding station arranged downstream of the
multiple coarse grinding stations.
[0018] In one aspect, the disclosure relates to a crystalline
material processing method comprising supplying emissions of a
laser focused within an interior of a crystalline material of a
substrate, and effecting relative lateral movement between the
laser and the substrate, to form subsurface laser damage having a
first subsurface laser damage pattern comprising a first plurality
of substantially parallel lines. The method further comprises,
following formation of the first subsurface laser damage pattern,
supplying laser emissions focused within the interior of the
crystalline material, and effecting relative lateral movement
between the laser and the substrate, to form subsurface laser
damage having a second subsurface laser damage pattern comprising a
second plurality of substantially parallel lines. According to such
method, lines of the second plurality of substantially parallel
lines are interleaved among lines of the first plurality of
substantially parallel lines, and at least some lines of the second
plurality of substantially parallel lines do not cross any lines of
the first plurality of substantially parallel lines.
[0019] In certain embodiments, each line of the second plurality of
substantially parallel lines does not cross any lines of the first
plurality of substantially parallel lines.
[0020] In certain embodiments, each line of the second plurality of
substantially parallel lines is arranged between a different pair
of adjacent lines of the first plurality of substantially parallel
lines.
[0021] In certain embodiments, the crystalline material comprises a
hexagonal crystal structure; and each line of the first plurality
of substantially parallel lines and each line of the second
plurality of substantially parallel lines is within .+-.5 degrees
of perpendicular to a <1120> direction of the hexagonal
crystal structure and substantially parallel to a surface of the
substrate.
[0022] In certain embodiments, spacing between at least some lines
of the first plurality of substantially parallel lines is
substantially the same as spacing between at least some lines of
the second plurality of substantially parallel lines.
[0023] In certain embodiments, each line of the second plurality of
substantially parallel lines does not cross any line of the first
plurality of substantially parallel lines.
[0024] In certain embodiments, the method further comprises,
following formation of the first subsurface laser damage pattern
and the second subsurface laser damage pattern, supplying laser
emissions focused within the interior of the crystalline material,
and effecting relative lateral movement between the laser and the
substrate, to form subsurface laser damage having a third
subsurface laser damage pattern comprising a third plurality of
substantially parallel lines. According to such method, lines of
the third plurality of substantially parallel lines are
interspersed or interleaved among lines of the first plurality of
substantially parallel lines and the second plurality of
substantially parallel lines.
[0025] In certain embodiments, each line of the third plurality of
substantially parallel lines is arranged between one line of the
first plurality of substantially parallel lines and one line of the
second plurality of substantially parallel lines.
[0026] In certain embodiments, a focusing depth of emissions of the
laser within the interior of the substrate differs among at least
two of the first, second, and third subsurface laser damage
patterns by a distance in a range of from about 2 microns to about
5 microns.
[0027] In certain embodiments, the first subsurface laser damage
pattern comprises a first plurality of cracks in the interior of
the crystalline material propagating laterally outward from lines
of the first plurality of substantially parallel lines; the second
subsurface laser damage pattern comprises a second plurality of
cracks in the interior of the crystalline material propagating
laterally outward from lines of the second plurality of
substantially parallel lines, and the second plurality of cracks is
non-connecting with the first plurality of cracks; and the third
subsurface laser damage pattern comprises a third plurality of
cracks in the interior of the crystalline material propagating
laterally outward from lines of the third plurality of
substantially parallel lines, wherein at least some cracks of the
third plurality of cracks connect with at least some cracks of the
first plurality of cracks and with at least some cracks of the
second plurality of cracks.
[0028] In certain embodiments, each line of the third plurality of
substantially parallel lines is arranged between a corresponding
line of the first plurality of substantially parallel lines and a
corresponding line of the second plurality of substantially
parallel lines to form a three-line group, such that the first,
second, and third subsurface laser damage patterns in combination
form a plurality of three-line groups; and for one or more
three-line groups of the plurality of three-line groups, the
three-line group is segregated from at least one adjacent
three-line group by an inter-group spacing that exceeds a spacing
between any two adjacent lines in the one or more three-line
groups.
[0029] In certain embodiments, the method further comprises,
following formation of the first, second, and third subsurface
laser damage patterns, supplying laser emissions focused within the
interior of the crystalline material, and effecting relative
lateral movement between the laser and the substrate, to form
subsurface laser damage having a fourth subsurface laser damage
pattern comprising a fourth plurality of substantially parallel
lines; wherein lines of the fourth plurality of substantially
parallel lines are interspersed or interleaved among lines of the
first, second, and third pluralities of substantially parallel
lines.
[0030] In certain embodiments, the crystalline material comprises a
hexagonal crystal structure; and each line of the first plurality
of substantially parallel lines, each line of the second plurality
of substantially parallel lines, each line of the third plurality
of substantially parallel lines deviates from perpendicular to a
<1120> direction of the hexagonal crystal structure by an
angle within a range of from about 1 degree to about 5 degrees
while being substantially parallel to a surface of the
substrate.
[0031] In certain embodiments, a focusing depth of emissions of the
laser within the interior of the substrate is substantially the
same during formation of the first subsurface laser damage pattern
and the second subsurface laser damage pattern.
[0032] In certain embodiments, at least some lines of the first
plurality of substantially parallel lines are arranged at
substantially the same depth within the interior of the crystalline
material as at least some lines of the second plurality of
substantially parallel lines.
[0033] In certain embodiments, the method further comprises:
detecting presence of a condition indicative of non-uniform doping
of the crystalline material across at least a portion of a surface
of the substrate, the non-uniform doping including at least one
first doping region and at least one second doping region; and
responsive to detection of the condition indicative of non-uniform
doping of the crystalline material, altering laser power to provide
laser emissions at a first average power when forming subsurface
laser damage in a first doping region and provide laser emissions
at a second average power when forming subsurface laser damage in a
second doping region, during formation of the first subsurface
laser damage pattern and the second subsurface laser damage
pattern.
[0034] In certain embodiments, the method further comprises
performing a repeat pass of at least one of the first, second, or
third subsurface laser damage patterns, comprising supplying laser
emissions focused within the interior of the crystalline material
to form a repeat subsurface laser damage pattern registered with at
least one of the first, second, or third subsurface laser damage
patterns, wherein the repeat subsurface damage pattern is centered
at a different depth relative to a surface of the crystalline
material than the at least one of the first, second, or third
subsurface laser damage patterns.
[0035] In certain embodiments, the crystalline material comprises a
single crystal semiconductor material.
[0036] In certain embodiments, wherein lines of the first plurality
of substantially parallel lines are non-parallel to lines of the
second plurality of substantially parallel lines, and an angular
direction of lines of the second plurality of substantially
parallel lines differs by no more than 10 degrees from an angular
direction of lines of the first plurality of substantially parallel
lines.
[0037] In certain embodiments, the method further comprises
fracturing the crystalline material substantially along at least
one of, or between, the first subsurface laser damage pattern and
the second subsurface laser damage pattern, to yield first and
second crystalline material portions each having reduced thickness
relative to the substrate, but substantially a same length and
width as the substrate.
[0038] In certain embodiments, at least one of the first
crystalline material portion or the second crystalline material
portion comprises a free-standing wafer configured for growth of at
least one epitaxial layer thereon. In certain embodiments, one of
the first crystalline material portion or the second crystalline
material portion comprises a device wafer including at least one
epitaxial layer grown thereon.
[0039] In another aspect, the disclosure relates to a crystalline
material processing method that comprises supplying emissions of a
laser focused within an interior of a substrate of crystalline
material, and effecting relative lateral movement between the laser
and the substrate, to form subsurface laser damage having an
initial subsurface laser damage pattern comprising an initial
plurality of substantially parallel lines; and supplying emissions
of the laser focused within the interior of the substrate, and
effecting relative lateral movement between the laser and the
substrate, to form subsurface laser damage having a subsequent
subsurface laser damage pattern comprising a subsequent plurality
of substantially parallel lines. According to such method, lines of
the initial plurality of substantially parallel lines are
non-parallel to lines of the subsequent plurality of substantially
parallel lines; an angular direction of lines of the subsequent
plurality of substantially parallel lines differs by no more than
10 degrees from an angular direction of lines of the initial
plurality of substantially parallel lines; and at least some lines
of the subsequent plurality of substantially parallel lines do not
cross any lines of the initial plurality of substantially parallel
lines.
[0040] In certain embodiments, each line of the subsequent
plurality of substantially parallel lines does not cross any lines
of the initial plurality of substantially parallel lines.
[0041] In certain embodiments, each line of the subsequent
plurality of substantially parallel lines is arranged between a
different pair of adjacent lines of the initial plurality of
substantially parallel lines.
[0042] In certain embodiments, the crystalline material comprises a
hexagonal crystal structure, each line of the initial plurality of
substantially parallel lines and each line of the subsequent
plurality of substantially parallel lines is within .+-.5 degrees
of perpendicular to a <1120> direction of the hexagonal
crystal structure and is substantially parallel to a surface of the
substrate.
[0043] In certain embodiments, lines of the subsequent plurality of
substantially parallel lines are interspersed or interleaved among
lines of the initial plurality of substantially parallel lines,
with each line of the subsequent plurality of substantially
parallel lines being arranged between a different pair of adjacent
lines of the initial plurality of substantially parallel lines.
[0044] In certain embodiments, one or more lines of the subsequent
plurality of substantially parallel lines cross one or more lines
of the initial plurality of substantially parallel lines.
[0045] In certain embodiments, the initial subsurface laser damage
pattern comprises a first subsurface laser damage pattern including
a first plurality of substantially parallel lines, and a second
subsurface laser damage pattern including a second plurality of
substantially parallel lines; the subsequent laser damage pattern
embodies a third subsurface laser damage pattern; and lines of the
third plurality of substantially parallel lines are interspersed or
interleaved among lines of the first plurality of substantially
parallel lines and the second plurality of substantially parallel
lines, with each line of the third plurality of substantially
parallel lines being arranged between one line of the first
plurality of substantially parallel lines and one line of the
second plurality of substantially parallel lines.
[0046] In certain embodiments, each line of the first plurality of
substantially parallel lines is separated from a nearest line of
the second plurality of substantially parallel lines by at least
100 microns.
[0047] In certain embodiments, a focusing depth of emissions of the
laser within the interior of the substrate is differs among at
least two of the first, second, and third subsurface laser damage
patterns by a distance in a range from about 2 microns to about 5
microns.
[0048] In certain embodiments, the first subsurface laser damage
pattern comprises a first plurality of cracks in the interior of
the crystalline material propagating laterally outward from lines
of the first plurality of substantially parallel lines; the second
subsurface laser damage pattern comprises a second plurality of
cracks in the interior of the crystalline material propagating
laterally outward from lines of the second plurality of
substantially parallel lines, and the second plurality of cracks is
non-connecting with the first plurality of cracks; and the third
subsurface laser damage pattern comprises a third plurality of
cracks in the interior of the crystalline material propagating
laterally outward from lines of the third plurality of
substantially parallel lines, wherein at least some cracks of the
third plurality of cracks connect with at least some cracks of the
first plurality of cracks and with at least some cracks of the
second plurality of cracks.
[0049] In certain embodiments, a focusing depth of emissions of the
laser within the interior of the substrate is substantially the
same during formation of the initial subsurface laser damage
pattern and the subsequent subsurface laser damage pattern.
[0050] In certain embodiments, the method further comprises:
detecting presence of a condition indicative of non-uniform doping
of the crystalline material across at least a portion of a surface
of the substrate, the non-uniform doping including at least one
first doping region and at least one second doping region; and
responsive to detection of the condition indicative of non-uniform
doping of the crystalline material, altering laser power to provide
laser emissions at a first average power when forming subsurface
laser damage in a first doping region and provide laser emissions
at a second average power when forming subsurface laser damage in a
second doping region, during formation of the initial subsurface
laser damage pattern and the subsequent subsurface laser damage
pattern.
[0051] In certain embodiments, the crystalline material comprises a
single crystal semiconductor material.
[0052] In certain embodiments, the method further comprises
fracturing the crystalline material substantially along at least
one of, or between, the initial subsurface laser damage pattern and
the subsequent subsurface laser damage pattern, to yield first and
second crystalline material portions each having reduced thickness
relative to the substrate, but substantially a same length and
width as the substrate.
[0053] In certain embodiments, at least one of the first
crystalline material portion or the second crystalline material
portion comprises a free-standing wafer configured for growth of at
least one epitaxial layer thereon. In certain embodiments, one of
the first crystalline material portion or the second crystalline
material portion comprises a device wafer including at least one
epitaxial layer grown thereon.
[0054] In another aspect, the disclosure relates to a crystalline
material processing method comprising: supplying laser emissions
focused to an initial depth within an interior of a crystalline
material of a substrate, and effecting relative lateral movement
between a laser and the substrate, to form subsurface laser damage
having an initial first subsurface laser damage pattern
substantially centered at the initial depth within the interior;
and supplying laser emissions focused to a subsequent within the
interior of the crystalline material, and effecting relative
lateral movement between the laser and the substrate, to form
subsurface laser damage having a subsequent laser damage pattern
substantially centered at the subsequent depth within the interior,
wherein the subsequent depth differs from the initial depth, the
subsequent subsurface laser damage pattern is substantially
registered with the initial subsurface laser damage pattern, and a
vertical extent of at least a portion of the subsurface laser
damage of the initial subsurface laser damage pattern overlaps with
a vertical extent of at least a portion of the subsurface laser
damage of the subsequent subsurface laser damage pattern.
[0055] In certain embodiments, a difference between the initial
depth and the subsequent depth is within a range of from about 2
microns to about 5 microns.
[0056] In certain embodiments, the crystalline material comprises a
hexagonal crystal structure, the initial subsurface laser damage
pattern comprises an initial plurality of substantially parallel
lines; the second subsurface laser damage pattern comprises a
subsequent plurality of substantially parallel lines; and each line
of the initial plurality of substantially parallel lines and each
line of the subsequent plurality of substantially parallel lines is
within .+-.5 degrees of perpendicular to a <1120> direction
of the hexagonal crystal structure and is substantially parallel to
a surface of the substrate.
[0057] In certain embodiments, lines of the subsequent plurality of
substantially parallel lines are non-crossing relative to lines of
the initial plurality of substantially parallel lines.
[0058] In certain embodiments, one or more lines of the subsequent
plurality of substantially parallel lines cross one or more lines
of the initial plurality of substantially parallel lines.
[0059] In certain embodiments, each of the initial subsurface laser
damage pattern and the subsequent laser damage pattern comprises a
first subsurface laser damage pattern including a first plurality
of substantially parallel lines, and a second subsurface laser
damage pattern including a second plurality of substantially
parallel lines; and lines of the first plurality of substantially
parallel lines are non-parallel to lines of the second plurality of
substantially parallel lines.
[0060] In certain embodiments, each line of the first plurality of
substantially parallel lines is separated from a nearest line of
the second plurality of substantially parallel lines by at least
100 microns.
[0061] In certain embodiments, the method further comprises:
detecting presence of a condition indicative of non-uniform doping
of the crystalline material across at least a portion of a surface
of the substrate, the non-uniform doping including at least one
first doping region and at least one second doping region; and
responsive to detection of the condition indicative of non-uniform
doping of the crystalline material, altering laser power to provide
laser emissions at a first average power when forming subsurface
laser damage in a first doping region and provide laser emissions
at a second average power when forming subsurface laser damage in a
second doping region, during formation of the initial subsurface
laser damage pattern and the subsequent subsurface laser damage
pattern.
[0062] In certain embodiments, the initial subsurface laser damage
pattern comprises an initial plurality of substantially parallel
lines; the second subsurface laser damage pattern comprises a
subsequent plurality of substantially parallel lines; lines of the
initial plurality of substantially parallel lines are non-parallel
to lines of the subsequent plurality of substantially parallel
lines; and no lines of the subsequent plurality of substantially
parallel lines are oriented more than 10 degrees apart from lines
of the initial plurality of substantially parallel lines.
[0063] In certain embodiments, the method further comprises
fracturing the crystalline material substantially along at least
one of, or between, the initial depth and the subsequent depth, to
yield first and second crystalline material portions each having
reduced thickness relative to the substrate, but substantially a
same length and width as the substrate.
[0064] In certain embodiments, at least one of the first
crystalline material portion or the second crystalline material
portion comprises a free-standing wafer configured for growth of at
least one epitaxial layer thereon. In certain embodiments, one of
the first crystalline material portion or the second crystalline
material portion comprises a device wafer including at least one
epitaxial layer grown thereon.
[0065] In another aspect, the disclosure relates to method for
processing a crystalline material that comprises a plurality of
areas that are non-overlapping relative to one another, the method
comprising: forming a first plurality of subsurface laser damage
regions in each area of the plurality of areas of the crystalline
material; and forming a second plurality of subsurface laser damage
regions in each area of the plurality of areas of the crystalline
material, wherein at least some subsurface laser damage regions of
the first plurality of subsurface laser damage regions do not cross
subsurface laser damage regions of the second plurality of
subsurface laser damage regions.
[0066] In certain embodiments, the first plurality of subsurface
laser damage regions comprises a first plurality of substantially
parallel lines, and the second plurality of subsurface laser damage
regions comprises a second plurality of substantially parallel
lines.
[0067] In certain embodiments, each line of the second plurality of
substantially parallel lines does not cross any lines of the first
plurality of substantially parallel lines. In certain embodiments,
each line of the second plurality of substantially parallel lines
is arranged between a different pair of adjacent lines of the first
plurality of substantially parallel lines.
[0068] In certain embodiments, spacing between at least some lines
of the first plurality of substantially parallel lines is
substantially the same as spacing between at least some lines of
the second plurality of substantially parallel lines.
[0069] In certain embodiments, the crystalline material comprises a
hexagonal crystal structure; and each line of the first plurality
of substantially parallel lines and each line of the second
plurality of substantially parallel lines is within .+-.5 degrees
of perpendicular to a <1120> direction of the hexagonal
crystal structure and substantially parallel to a flat surface of
the crystalline material.
[0070] In certain embodiments, the method further comprises,
following formation of the first plurality of subsurface laser
damage regions and the second subsurface laser damage regions,
forming a third plurality of subsurface laser damage regions in
each area of the plurality of areas of the crystalline
material.
[0071] In certain embodiments, the first plurality of subsurface
laser damage regions comprises a first plurality of substantially
parallel lines; the second plurality of subsurface laser damage
regions comprises a second plurality of substantially parallel
lines; the third plurality of subsurface laser damage regions
comprises a third plurality of substantially parallel lines; and at
least some lines of the third plurality of substantially parallel
lines are interspersed among lines of the first plurality of
substantially parallel lines and the second plurality of
substantially parallel lines.
[0072] In certain embodiments, the method further comprises
repeating formation of at least one of the first, second, or third
subsurface laser damage region to form a repeat subsurface laser
damage region registered with at least one of the first, second, or
third subsurface laser damage patterns, wherein the repeat
subsurface damage region is centered at a different depth relative
to a surface of the crystalline material than the at least one of
the first, second, or third subsurface laser damage patterns.
[0073] In certain embodiments, each laser damage region extends
substantially from one lateral boundary of the crystalline material
to another lateral boundary of the crystalline material.
[0074] In certain embodiments, the plurality of areas comprises at
least three areas.
[0075] In certain embodiments, the crystalline material comprises a
single crystal semiconductor material.
[0076] In certain embodiments, the crystalline material comprises a
substrate, and the method further comprises fracturing the
crystalline material substantially along at least one of, or
between, the first plurality of subsurface laser damage regions and
the second plurality of subsurface laser damage regions, to yield
first and second crystalline material portions each having reduced
thickness relative to the substrate, but substantially a same
length and width as the substrate.
[0077] In certain embodiments, at least one of the first
crystalline material portion or the second crystalline material
portion comprises a free-standing wafer configured for growth of at
least one epitaxial layer thereon.
[0078] In certain embodiments, one of the first crystalline
material portion or the second crystalline material portion
comprises a device wafer including at least one epitaxial layer
grown thereon.
[0079] In another aspect, the disclosure relates to a method for
processing a crystalline material that comprises a plurality of
areas that are non-overlapping relative to one another, the method
comprising: sequentially forming first and second pluralities of
subsurface laser damage regions across each area of the plurality
of areas to form interspersed subsurface laser damage regions,
wherein at least some subsurface laser damage regions of the first
plurality of subsurface laser damage regions do not cross
subsurface laser damage regions of the second plurality of
subsurface laser damage regions.
[0080] In certain embodiments, the first plurality of subsurface
laser damage regions comprises a first plurality of substantially
parallel lines, and the second plurality of subsurface laser damage
regions comprises a second plurality of substantially parallel
lines.
[0081] In certain embodiments, each line of the second plurality of
substantially parallel lines does not cross any lines of the first
plurality of substantially parallel lines.
[0082] In certain embodiments, each line of the second plurality of
substantially parallel lines is arranged between a different pair
of adjacent lines of the first plurality of substantially parallel
lines.
[0083] In certain embodiments, spacing between at least some lines
of the first plurality of substantially parallel lines is
substantially the same as spacing between at least some lines of
the second plurality of substantially parallel lines.
[0084] In certain embodiments, the crystalline material comprises a
hexagonal crystal structure; and each line of the first plurality
of substantially parallel lines and each line of the second
plurality of substantially parallel lines is within .+-.5 degrees
of perpendicular to a <1120> direction of the hexagonal
crystal structure and substantially parallel to a flat surface of
the crystalline material.
[0085] In certain embodiments, the method further comprises,
following formation of the first plurality of subsurface laser
damage regions and the second plurality of subsurface laser damage
regions, forming a third plurality of subsurface laser damage
regions in each area of the plurality of areas of the crystalline
material.
[0086] In certain embodiments, the first plurality of subsurface
laser damage regions comprises a first plurality of substantially
parallel lines; the second plurality of subsurface laser damage
regions comprises a second plurality of substantially parallel
lines; the third plurality of subsurface laser damage regions
comprises a third plurality of substantially parallel lines; and at
least some lines of the third plurality of substantially parallel
lines are interspersed among lines of the first plurality of
substantially parallel lines and the second plurality of
substantially parallel lines.
[0087] In certain embodiments, the method further comprises
repeating formation of at least one of the first, second, or third
subsurface laser damage region to form a repeat subsurface laser
damage region registered with at least one of the first, second, or
third subsurface laser damage patterns, wherein the repeat
subsurface damage region is centered at a different depth relative
to a surface of the crystalline material than the at least one of
the first, second, or third subsurface laser damage patterns.
[0088] In certain embodiments, each laser damage region extends
substantially from one lateral boundary of the crystalline material
to another lateral boundary of the crystalline material.
[0089] In certain embodiments, the plurality of areas comprises at
least three areas.
[0090] In certain embodiments, the crystalline material comprises a
single crystal semiconductor material.
[0091] In certain embodiments, the crystalline material comprises a
substrate, and the method further comprises fracturing the
crystalline material substantially along at least one of, or
between, the first plurality of subsurface laser damage regions and
the second plurality of subsurface laser damage regions, to yield
first and second crystalline material portions each having reduced
thickness relative to the substrate, but substantially a same
length and width as the substrate.
[0092] In certain embodiments, at least one of the first
crystalline material portion or the second crystalline material
portion comprises a free-standing wafer configured for growth of at
least one epitaxial layer thereon.
[0093] In certain embodiments, one of the first crystalline
material portion or the second crystalline material portion
comprises a device wafer including at least one epitaxial layer
grown thereon.
[0094] In another aspect, the disclosure relates to a method for
processing a crystalline material wafer comprising a first surface
having surface damage thereon, the first surface being bounded by
an edge, the method comprising: grinding the first surface with at
least one first grinding apparatus to remove a first part of the
surface damage; following the grinding of the first surface with
the at least one first grinding apparatus, edge grinding the edge
to form a beveled or rounded edge profile; and following the edge
grinding, grinding the first surface with at least one second
grinding apparatus to remove a second part of the surface damage
sufficient to render the first surface suitable for further
processing by chemical mechanical planarization.
[0095] In certain embodiments, the method further comprises
processing the first surface by chemical mechanical planarization
following the grinding of the first surface with the at least one
second grinding apparatus to render the first surface for epitaxial
growth of one or more layers of semiconductor material thereon.
[0096] In certain embodiments, the at least one first grinding
apparatus comprises at least one grinding wheel having a grinding
surface of less than 5000 grit (e.g., 1000 grit, 1400 grit, 2000
grit, 3000 grit, 4000 grit, or the like), and the at least one
second grinding apparatus comprises at least one grinding wheel
having a grinding surface of at least 5000 grit (e.g., 5000 grit,
7000 grit, 8000 grit, 10,000 grit, 15,000 grit, 20,000 grit, 25,000
grit, 30,000 grit, or the like).
[0097] In certain embodiments, the grinding of the first surface
with the at least one first grinding apparatus comprises removal of
a thickness of 20 microns to 100 microns (e.g., 20 microns to 80
microns, 40 microns to 80 microns, 40 to 60 microns, or the like)
of the crystalline material, and the grinding of the second surface
with the at least one second grinding apparatus comprises removal
of a thickness of 3 to 15 microns (e.g., 5 to 10 microns) of the
crystalline material.
[0098] In certain embodiments, the surface damage comprises laser
damage and fracture damage.
[0099] In certain embodiments, the crystalline material comprises
silicon carbide material, and the first surface comprises a
Si-terminated face of the silicon carbide material.
[0100] In another aspect, the disclosure relates to a method for
processing a crystalline material wafer comprising a first surface
having surface damage thereon, the first surface being bounded by
an edge, the method comprising: grinding the first surface with at
least one first grinding apparatus to remove a first part of the
surface damage; following the grinding of the first surface with
the at least one first grinding apparatus, grinding the first
surface with at least one second grinding apparatus to remove a
second part of the surface damage sufficient to render the first
surface suitable for further processing by chemical mechanical
planarization; following the grinding of the first surface with the
at least one second grinding apparatus, forming a protective
coating on the first surface; following the deposition of the
sacrificial material on the first surface, edge grinding the edge
to form a beveled or rounded edge profile; and following the edge
grinding, removing the protective coating from the first
surface.
[0101] In certain embodiments, the method further comprises
processing the first surface by chemical mechanical planarization
following the removal of the sacrificial material from the first
surface, to render the first surface for epitaxial growth of one or
more layers of semiconductor material thereon.
[0102] In certain embodiments, the at least one first grinding
apparatus comprises at least one grinding wheel having a grinding
surface of less than 5000 grit, and the at least one second
grinding apparatus comprises at least one grinding wheel having a
grinding surface of at least 5000 grit.
[0103] In certain embodiments, the grinding of the first surface
with the at least one first grinding apparatus comprises removal of
a thickness of 20 microns to 100 microns of the crystalline
material, and the grinding of the second surface with the at least
one second grinding apparatus comprises removal of a thickness of 3
to 15 microns of the crystalline material.
[0104] In certain embodiments, the protective coating comprises
photoresist.
[0105] In certain embodiments, the surface damage comprises laser
damage and fracture damage.
[0106] In certain embodiments, the crystalline material comprises
silicon carbide material, and the first surface comprises a
Si-terminated face of the silicon carbide material.
[0107] In another aspect, the disclosure relates to a material
processing apparatus comprising: a laser processing station
configured to form subsurface laser damage regions in crystalline
material substrates supplied to the laser processing station; a
fracturing station arranged to receive crystalline material
substrates processed by the laser processing station and configured
to fracture the crystalline material substrates along the
subsurface laser damage regions to form crystalline material
portions removed from the crystalline material substrates, wherein
each crystalline material portion comprises surface damage; a
plurality of coarse grinding stations arranged in parallel
downstream of the fracturing station and configured to remove first
parts of the surface damage from the crystalline material portions,
wherein at least first and second coarse grinding stations of the
plurality of coarse grinding stations are configured to be operated
simultaneously to remove first parts of surface damage of different
crystalline material portions; and at least one fine grinding
station arranged downstream of the plurality of coarse grinding
stations and configured to remove second parts of the surface
damage from the crystalline material portions, sufficient to render
at least one surface of each crystalline material portion suitable
for further processing by chemical mechanical planarization.
[0108] In certain embodiments, the apparatus further comprises at
least one chemical mechanical planarization station arranged
downstream of the at least one fine grinding station and configured
to render at least one surface of each crystalline material portion
suitable for further processing by chemical mechanical
planarization.
[0109] In certain embodiments, the apparatus further comprises at
least one edge grinding station configured to grind an edge of each
crystalline material portion to form a beveled or rounded edge
profile.
[0110] In certain embodiments, each coarse grinding station
comprises at least one grinding wheel having a grinding surface of
less than 5000 grit, and the at least one fine grinding station
comprises at least one grinding wheel having a grinding surface of
at least 5000 grit.
[0111] In certain embodiments, each coarse grinding station is
configured to remove a thickness of 20 microns to 100 microns of
crystalline material from each crystalline material portion, and
each fine grinding station is configured to remove a thickness of 3
to 15 microns of crystalline material from each crystalline
material portion.
[0112] In certain embodiments, the laser processing station is
configured to simultaneously form subsurface laser damage regions
in multiple crystalline material substrates.
[0113] In another aspect, any of the foregoing aspects, and/or
various separate aspects and features as described herein, may be
combined for additional advantage. Any of the various features and
elements as disclosed herein may be combined with one or more other
disclosed features and elements unless indicated to the contrary
herein.
[0114] Other aspects, features and embodiments of the present
disclosure will be more fully apparent from the ensuing disclosure
and appended claims.
BRIEF DESCRIPTION OF DRAWINGS
[0115] The accompanying drawings incorporated in and forming a part
of this specification illustrate several aspects of the disclosure,
and together with the description serve to explain the principles
of the disclosure.
[0116] FIG. 1 includes a first frame providing a perspective view
of an ingot received by a conventional wire saw tool and being
subjected to a wire sawing process, and a second frame providing a
perspective view of multiple wafers obtained by the wire sawing
process.
[0117] FIG. 2 is a first perspective view crystal plane diagram
showing the coordinate system for a hexagonal crystal such as
4H-SiC.
[0118] FIG. 3 is a second perspective view crystal plane diagram
for a hexagonal crystal, illustrating a vicinal plane that is
non-parallel to the c-plane.
[0119] FIG. 4A is a perspective view wafer orientation diagram
showing orientation of a vicinal wafer relative to the c-plane.
[0120] FIG. 4B is a simplified cross-sectional view of the vicinal
wafer of FIG. 4A superimposed over a portion of an ingot.
[0121] FIG. 5 is a top plan view of an exemplary SiC wafer, with
superimposed arrows showing crystallographic orientation
directions.
[0122] FIG. 6A is a side elevation schematic view of an on-axis
ingot of crystalline material.
[0123] FIG. 6B is a side elevation schematic view of the ingot of
FIG. 6A being rotated by 4 degrees, with a superimposed pattern for
cutting end portions of the ingot.
[0124] FIG. 6C is a side elevation schematic view of an ingot
following removal of end portions to provide end faces that are
non-perpendicular to the c-direction
[0125] FIG. 7 is a perspective view schematic of a moveable laser
tool configured to focus laser emissions within an interior of a
crystalline material to form subsurface damage.
[0126] FIGS. 8A and 8B provide exemplary laser tool travel paths
relative to a crystalline material for formation of subsurface
damage within the crystalline material, with FIG. 8B including a
superimposed arrow showing orientation of subsurface damage lines
relative to the [1120] direction of a hexagonal crystal structure
of the crystalline material.
[0127] FIG. 9 is a perspective view schematic of the surface
structure of an off-axis (relative to the c-axis) or vicinal 4H-SiC
crystal after fracture but prior to smoothing, with the fractured
surface exhibiting terraces and steps.
[0128] FIGS. 10A-10D are cross-sectional schematic views of
formation of subsurface laser damage in a substrate of crystalline
material by focusing laser emissions into a bare substrate, through
a surface of a substrate supported by a carrier, through a carrier
and an adhesive layer into a substrate, and through a carrier into
a substrate, respectively.
[0129] FIG. 11A provides a top plan view of a crystalline material
substrate including interspersed first, second, and third
subsurface laser damage patterns defined therein according to one
embodiment, with each damage pattern including a plurality of
substantially parallel lines perpendicular to the [1120] direction
(and substantially perpendicular to a primary substrate flat), and
with the laser damage patterns in combination forming multiple
three-line groups that are separated from one another by an
inter-group spacing that exceeds a spacing between adjacent lines
in each three-line group.
[0130] FIG. 11B is a top plan schematic view of the crystalline
material substrate of FIG. 11A during fabrication, following
formation of the first subsurface laser damage pattern, with
illustration of a first plurality of cracks within the interior of
the substrate propagating laterally outward from the first
plurality of substantially parallel lines.
[0131] FIG. 11C is a top plan view of the crystalline material
substrate of FIG. 11B, upon formation of the second subsurface
laser damage pattern after the first subsurface laser damage
pattern, with illustration of a second plurality of cracks within
the interior of the substrate propagating laterally outward from
the second plurality of substantially parallel lines but not
contacting the first plurality of cracks.
[0132] FIG. 11D is a top plan view of the crystalline material
substrate of FIG. 11C, upon formation of the third subsurface laser
damage pattern after the first and second subsurface laser damage
patterns, with illustration of a third plurality of cracks within
the interior of the substrate propagating laterally outward from
the third plurality of substantially parallel lines and connecting
cracks of the first plurality and second plurality of cracks.
[0133] FIG. 12 is a top plan schematic view of a crystalline
material substrate including interspersed first through third
subsurface laser damage patterns defined therein according to one
embodiment similar to that shown in FIG. 11A, with each damage
pattern including a plurality of substantially parallel lines that
deviate three degrees relative to perpendicular to the [1120]
direction along the substrate surface (and substantially
perpendicular to a primary substrate flat), and with the laser
damage patterns in combination forming multiple three-line groups
that are separated from one another by an inter-group spacing that
exceeds a spacing between adjacent lines in each three-line
group.
[0134] FIG. 13 is a top plan schematic view of a crystalline
material substrate including interspersed first through fourth
laser damage patterns with all lines parallel to one another and
perpendicular to the [1120] direction along the substrate surface
(and substantially perpendicular to a primary substrate flat).
[0135] FIG. 14 is a top plan schematic view of a crystalline
material substrate including interspersed first, second, and third
subsurface laser damage patterns defined therein according to one
embodiment in which first and second groups of lines are each
parallel to one another and perpendicular to the [1120] direction
along the substrate surface (and substantially perpendicular to a
primary substrate flat), and the third group of lines is
non-parallel to the first and second groups of lines but does not
cross lines of the first and second groups of lines within the
substrate.
[0136] FIG. 15 is a top plan schematic view of a crystalline
material substrate including interspersed first, second, and third
subsurface laser damage patterns defined therein according to one
embodiment in which first and second groups of lines are each
parallel to one another and deviate about 3 degrees from
perpendicular to the [1120] direction along the substrate surface
(and substantially perpendicular to a primary substrate flat), and
the third group of lines is perpendicular to the primary substrate
flat but does not cross lines of the first and second groups of
lines within the substrate.
[0137] FIG. 16 is a top plan schematic view of a crystalline
material substrate including interspersed first, second, and third
subsurface laser damage patterns defined therein according to one
embodiment in which all laser damage lines are parallel to one
another, and inter-group spacing of laser damage lines is not
uniform over at least portions of the substrate.
[0138] FIG. 17 is a top plan schematic view of a crystalline
material substrate including interspersed first, second, and third
subsurface laser damage patterns defined therein according to one
embodiment in which all laser damage lines are parallel to one
another, and laser damage lines exhibit variation in intra-group
spacing, inter-group spacing, and group composition.
[0139] FIG. 18 is a top plan schematic view of a crystalline
material substrate including sequentially formed first, second, and
third subsurface laser damage patterns defined therein according to
one embodiment in which first and second groups of laser damage
lines are parallel to one another, while a third group of laser
damage lines are non-parallel to and cross the first and second
groups of laser damage lines.
[0140] FIG. 19 is a top plan schematic view of a crystalline
material substrate including sequentially formed first, second, and
third subsurface laser damage patterns in which each group of laser
damage lines includes parallel lines, and each group of laser
damage lines is non-parallel to each other group of laser damage
lines.
[0141] FIG. 20A is a top plan view of a crystalline material
substrate illustrating non-overlapping first, second, and third
areas in which laser damage regions may be formed.
[0142] FIG. 20B is a top plan view of the crystalline material
substrate of FIG. 20A following formation of a first plurality of
subsurface laser damage regions in the first through third
areas.
[0143] FIG. 20C is a top plan view of the crystalline material
substrate of FIG. 20B following formation of a second plurality of
subsurface laser damage regions in the first through third
areas.
[0144] FIG. 20D is a top plan view of the crystalline material
substrate of FIG. 20C following formation of a third plurality of
subsurface laser damage regions in the first through third
areas.
[0145] FIG. 21 is a top plan schematic view of a holder of a laser
processing apparatus arranged to hold four substrates in which
subsurface laser damage may be formed with one or more lasers.
[0146] FIG. 22A is a top plan schematic view of a single substrate
being processed with a split laser beam to simultaneously form
subsurface laser damage according to a first subsurface laser
damage pattern in two portions of the substrate.
[0147] FIG. 22B is a top plan schematic view of a two substrates
being processed with a split laser beam to simultaneously form
subsurface laser damage according to a first subsurface laser
damage pattern in both substrates.
[0148] FIG. 23A is a cross-sectional schematic view of a
crystalline material substrate including a first subsurface laser
damage pattern centered at a first depth.
[0149] FIG. 23B is a cross-sectional schematic view of the
substrate of FIG. 23A following formation of a second subsurface
laser damage pattern centered at a second depth and registered with
the first subsurface laser damage pattern, with an overlapping
vertical extent of the first and second damage patterns.
[0150] FIG. 24A is a perspective view photograph of a SiC wafer
following separation from a thermoplastic glue-bonded sapphire
carrier according to a method described herein.
[0151] FIG. 24B is a perspective view photograph of the sapphire
carrier from which the SiC wafer of FIG. 24A was separated.
[0152] FIG. 24C is a partially tone-reversed version of the SiC
wafer photograph of FIG. 24A to emphasize contrast between a
central doping ring and an annular outer portion of the wafer.
[0153] FIG. 24D shows the image of FIG. 24C annotated with a
dashed-line oval to denote a boundary between the central doping
ring and the annular outer portion of the wafer.
[0154] FIG. 25 is a perspective view photograph of a Si face of a
SiC wafer separated from an ingot by a process involving formation
of subsurface laser damage and subsequent separation, with an inset
portion (upper right) depicting a fragment of the SiC wafer
including an edge depicted in subsequent scanning electron
microscope (SEM) images.
[0155] FIG. 26 is a 45 times magnification SEM image, taken at a 15
degree tilt angle, of a portion of the SiC wafer fragment of FIG.
25, with superimposed arrows showing directions of the [1100] and
[1120] crystallographic planes.
[0156] FIG. 27 is a 1,300 times magnification SEM image, taken at a
15 degree tilt angle, of a portion of the SiC wafer fragment of
FIG. 25.
[0157] FIG. 28 is a 350 times magnification SEM image, taken at a
15 degree tilt angle, of a portion of the SiC wafer fragment of
FIG. 25.
[0158] FIG. 29 is a 100 times magnification SEM image taken at a 2
degree tilt angle, of a portion of the SiC wafer fragment of FIG.
25.
[0159] FIG. 30 is a 1,000 times magnification SEM image taken at a
2 degree tilt angle, of a portion of the SiC wafer fragment of FIG.
25.
[0160] FIG. 31A is a confocal laser scanning microscopy image of a
small, central portion of the SiC wafer of FIG. 25, with
superimposed crosshairs marking positions of "trenches" formed by
laser scanning.
[0161] FIG. 31B is a surface profile plot of the portion of the SiC
wafer of FIG. 21A.
[0162] FIG. 32A is a confocal laser scanning microscopy image of a
larger, top-proximate (as pictured) portion of the SiC wafer of
FIG. 25, with superimposed crosshairs marking positions of
"trenches" formed by laser scanning.
[0163] FIG. 32B is a surface profile plot of the top-proximate
portion of the SiC wafer of FIG. 32A.
[0164] FIG. 33A is a confocal laser scanning microscopy image of a
larger, bottom-proximate (as pictured) portion of the SiC wafer of
FIG. 25, with superimposed crosshairs marking positions of
"trenches" formed by laser scanning.
[0165] FIG. 33B is a surface profile plot of the bottom-proximate
portion of the SiC wafer of FIG. 33A.
[0166] FIG. 34A is a side cross-sectional schematic view of a solid
carrier having adhesive material joined to a surface thereof.
[0167] FIG. 34B is a cross-sectional schematic view of an assembly
including the solid carrier and adhesive material of FIG. 34A
joined to a crystalline material substrate having a subsurface
laser damage region proximate to the adhesive material lip.
[0168] FIG. 34C is a cross-sectional schematic view of the assembly
of FIG. 34B, with a surface of the solid carrier being positioned
on a cooling apparatus in the form of a liquid-cooled chuck.
[0169] FIG. 34D is a cross-sectional schematic view of a majority
of the crystalline material substrate separated from a bonded
assembly (atop the liquid-cooled chuck) including the solid carrier
and a portion of the crystalline material removed from the
substrate, following fracture of the crystalline material along the
subsurface laser damage region.
[0170] FIG. 34E is a cross-sectional schematic view of the bonded
assembly of FIG. 34D following removal from the liquid-cooled
chuck, with residual laser damage along an upward facing
surface.
[0171] FIG. 34F is a cross-sectional schematic view of the portion
of the crystalline material supported by a heated vacuum chuck,
with the solid carrier and adhesive material being laterally
translated away from the crystalline material portion following
thermal softening and release of the adhesive material.
[0172] FIG. 35 is a cross-sectional schematic view of a crystalline
material having subsurface laser damage and bonded to a rigid
carrier, with the crystalline material and carrier arranged in a
liquid bath of an ultrasonic generator.
[0173] FIGS. 36A-36C are cross-sectional schematic views
illustrating steps for fracturing a crystalline material having
subsurface laser damage including application of a mechanical force
proximate to one edge of a carrier to impart a bending moment in at
least a portion of the carrier.
[0174] FIGS. 37A-37O are cross-sectional schematic views
illustrating steps of a device wafer splitting process, according
to which a thick wafer is fractured from a crystalline material, at
least one epitaxial layer is grown on the thick wafer, and the
thick wafer is fractured to form a first and second bonded
assemblies each including a carrier and a thin wafer divided from
the thick wafer, with the first bonded assembly including the at
least one epitaxial layer as part of an operative
semiconductor-based device.
[0175] FIG. 38 is a flowchart schematically illustrating steps for
producing subsurface laser damage and bonding a rigid carrier to a
crystalline (e.g., SiC) material ingot, followed by laser parting
of a bonded assembly including the carrier and a portion of the
crystalline material, followed by further processing of the bonded
assembly and formation of epitaxial layers on a device wafer, with
return of the ingot and the rigid carrier to a beginning of the
process.
[0176] FIG. 39 is a cross-sectional schematic view of a portion of
the crystalline material substrate of FIG. 38 showing subsurface
laser damage with superimposed dashed lines identifying an
anticipated kerf loss material region attributable to laser damage
and subsequent surface processing (e.g., grinding and
planarization).
[0177] FIG. 40 is a schematic illustration of a material processing
apparatus according to one embodiment, including a laser processing
station, a material fracturing station, multiple coarse grinding
stations arranged in parallel, a fine grinding station, and a CMP
station.
[0178] FIG. 41 is a schematic illustration of a material processing
apparatus according to one embodiment similar to that of FIG. 40,
but with an edge grinding station arranged between the fine
grinding station and the coarse grinding stations.
[0179] FIG. 42 is a schematic illustration of a material processing
apparatus according to one embodiment, including a laser processing
station, a material fracturing station, multiple coarse grinding
stations arranged in parallel, a fine grinding station, a surface
coating station, an edge grinding station, a coating removal
station, and a CMP station.
[0180] FIG. 43A is a schematic side cross-sectional view of a first
apparatus for holding an ingot having end faces that are
non-perpendicular to a sidewall thereof, according to one
embodiment.
[0181] FIG. 43B is a schematic side cross-sectional view of a
second apparatus for holding an ingot having end faces that are
non-perpendicular to a sidewall thereof, according to one
embodiment.
DETAILED DESCRIPTION
[0182] Aspects of the present disclosure provide methods for
processing a crystalline material substrate to form multiple
patterns of subsurface laser damage that facilitate subsequent
fracture of the substrate to yield reduced thickness first and
second crystalline material portions of the substrate. Certain
methods involve interspersing of multiple sequentially formed
pluralities of substantially parallel lines of multiple subsurface
laser damage patterns, respectively, wherein at least some lines of
a second (e.g., subsequently formed) plurality of lines do not
cross lines of a first plurality of lines. Certain methods involve
formation of initial and subsequent subsurface laser damage
patterns each comprising a plurality of substantially parallel
lines in a substrate of crystalline material, with lines of the
initial and subsequent pluralities of substantially parallel lines
being non-parallel to one another, wherein an angular direction of
lines of the subsequent plurality of substantially parallel lines
differs by no more than 10 degrees from an angular direction of
lines of the initial plurality of substantially parallel lines, and
at least some lines of the subsequent plurality of substantially
parallel lines do not cross any lines of the initial plurality of
substantially parallel lines. . Certain methods involve formation
of an initial subsurface laser damage pattern substantially
centered at an initial depth within an interior of a crystalline
material of a substrate, and formation of a subsequent subsurface
laser damage pattern substantially centered at a subsequent depth
(differing from the initial depth) within the substrate, wherein
the subsequent subsurface laser damage pattern is substantially
registered with the initial subsurface laser damage pattern, and
vertical extents of at least portions of the initial and subsurface
laser damage patterns are overlapping.
[0183] Sequential formation of interspersed or interleaved
subsurface laser damage patterns distributed over a crystalline
material is believed to beneficially maintain sufficient stress
within a crystalline material to facilitate subsequent material
fracture using methods herein, while enabling high laser tool
throughput in conjunction with modest material damage and
concomitantly low kerf losses. It would be simple in principle to
use high laser power and scan nearly an entirety of a crystalline
material to facilitate fracturing along a laser damage line. Such
an approach can reliably separate thin layers of crystalline
material from a bulk substrate (e.g., an ingot), but high laser
power tends to increase material damage, necessitating significant
surface processing (e.g., grinding and planarization) to remove the
damage. Close spacing between laser damage lines will help promote
fracture, but at the cost of significantly reducing throughput of a
laser processing tool. A conventional approach for forming
subsurface laser damage has involved forming a subsurface laser
damage line in a forward direction across a crystalline material,
followed by relative indexing in a lateral direction between the
material and a laser, followed by forming a subsurface laser damage
line in a rearward direction, followed by lateral indexing in the
same lateral direction, and so on. Such approach generally requires
higher laser power or closer spacing between sequentially formed
laser damage lines, which will tend to reduce throughput or impart
a greater degree of damage, thereby increasing kerf loss due to the
need to remove additional material from laser-processed surfaces
for removal of the laser damage. This conventional approach does
not involve forming a first distributed subsurface laser damage
pattern (e.g., involving formation of a first plurality of laser
damage regions over multiple non-overlapping areas of a substrate)
followed by formation of a second distributed subsurface laser
damage pattern (e.g., involving formation of a second plurality of
laser damage regions over the same multiple non-overlapping areas
of the substrate), with the second subsurface laser damage pattern
being interleaved or interspersed among the first subsurface laser
damage pattern.
[0184] Various embodiments disclosed herein address the concern of
promoting reliable separation of thin layers (e.g., wafers) of
crystalline material from a substrate without unduly high laser
power, while enabling high laser tool throughput and providing low
kerf losses. Certain embodiments herein involve forming an initial
distributed subsurface laser damage pattern in a crystalline
material substrate (e.g., over each area of a plurality of
non-overlapping areas of the substrate), then forming at least one
subsequent distributed subsurface laser damage pattern in the same
substrate (e.g., over each area of the same plurality of
non-overlapping areas), wherein at least portions (e.g., lines) of
the at least one subsequent laser damage pattern are arranged in
gaps between laser damage lines of the initial laser damage
pattern, thereby providing interspersed or interleaved subsurface
laser damage patterns. In certain embodiments, at least some (or
all) laser damage lines of at least one subsequently formed laser
damage pattern do not cross laser damage lines of an initial
subsurface laser damage pattern. It is believed that non-crossing
of laser damage patterns may beneficially avoid localized stresses
from being dissipated. In certain embodiments, first and second
interspersed subsurface laser damage patterns are formed in such a
manner to prevent propagation of localized subsurface cracks
therebetween, but application of a third (or subsequent)
interspersed subsurface laser damage pattern will cause localized
subsurface cracks to propagate and join in a substantially
continuous manner over an entire internal plane of a crystalline
material substrate, thereby easing subsequent fracture along the
laser damage region using techniques disclosed herein. Formation of
interspersed subsurface laser damage according to methods described
herein has been observed to permit reliable separation of thin
layers of crystalline material from a substrate with a smaller
number of laser damage lines per layer to be removed, beneficially
providing increased laser tool throughput while providing low
levels of laser damage (enabling low kerf losses).
[0185] The embodiments set forth below represent the necessary
information to enable those skilled in the art to practice the
embodiments and illustrate the best mode of practicing the
embodiments. Upon reading the following description in light of the
accompanying drawings, those skilled in the art will understand the
concepts of the disclosure and will recognize applications of these
concepts not particularly addressed herein. It should be understood
that these concepts and applications fall within the scope of the
disclosure and the accompanying claims.
[0186] It will be understood that, although the terms first,
second, etc. may be used herein to describe various elements, these
elements should not be limited by these terms. These terms are only
used to distinguish one element from another. For example, a first
element could be termed a second element, and, similarly, a second
element could be termed a first element, without departing from the
scope of the present disclosure. As used herein, the term "and/or"
includes any and all combinations of one or more of the associated
listed items.
[0187] It will be understood that when an element such as a layer,
region, or substrate is referred to as being "on" or extending
"onto" another element, it can be directly on or extend directly
onto the other element or intervening elements may also be present.
In contrast, when an element is referred to as being "directly on"
or extending "directly onto" another element, there are no
intervening elements present. Likewise, it will be understood that
when an element such as a layer, region, or substrate is referred
to as being "over" or extending "over" another element, it can be
directly over or extend directly over the other element or
intervening elements may also be present. In contrast, when an
element is referred to as being "directly over" or extending
"directly over" another element, there are no intervening elements
present. It will also be understood that when an element is
referred to as being "connected" or "coupled" to another element,
it can be directly connected or coupled to the other element or
intervening elements may be present. In contrast, when an element
is referred to as being "directly connected" or "directly coupled"
to another element, there are no intervening elements present.
[0188] Relative terms such as "below" or "above" or "upper" or
"lower" or "horizontal" or "vertical" may be used herein to
describe a relationship of one element, layer, or region to another
element, layer, or region as illustrated in the Figures. It will be
understood that these terms and those discussed above are intended
to encompass different orientations of the device in addition to
the orientation depicted in the Figures.
[0189] The terminology used herein is for the purpose of describing
particular embodiments only and is not intended to be limiting of
the disclosure. As used herein, the singular forms "a," "an," and
"the" are intended to include the plural forms as well, unless the
context clearly indicates otherwise. It will be further understood
that the terms "comprises," "comprising," "includes," and/or
"including" when used herein specify the presence of stated
features, integers, steps, operations, elements, and/or components,
but do not preclude the presence or addition of one or more other
features, integers, steps, operations, elements, components, and/or
groups thereof.
[0190] Unless otherwise defined, all terms (including technical and
scientific terms) used herein have the same meaning as commonly
understood by one of ordinary skill in the art to which this
disclosure belongs. It will be further understood that terms used
herein should be interpreted as having a meaning that is consistent
with their meaning in the context of this specification and the
relevant art and will not be interpreted in an idealized or overly
formal sense unless expressly so defined herein.
[0191] As used herein, a "substrate" refers to a crystalline
material, such as a single crystal semiconductor material,
optionally comprising an ingot or a wafer, that is divisible into
at least two thinner portions having substantially the same lateral
dimensions (e.g., diameter, or length and width) as the substrate,
and having sufficient thickness (i) to be surface processed (e.g.,
lapped and polished) to support epitaxial deposition of one or more
semiconductor material layers, and optionally (ii) to be
free-standing if and when separated from a rigid carrier. In
certain embodiments, a substrate may have a generally cylindrical
shape, and/or may have a thickness of at least about one or more of
the following thicknesses: 300 .mu.m, 350 .mu.m, 500 .mu.m, 750
.mu.m, 1 mm, 2 mm, 3 mm, 5 mm, 1 cm, 2 cm, 5 cm, 10 cm, 20 cm, 30
cm, or more. In certain embodiments, a substrate may include a
thicker wafer that is divisible into two thinner wafers. In certain
embodiments, a substrate may be part of a thicker wafer having one
or more epitaxial layers (optionally in conjunction with one or
more metal contacts) arranged thereon as part of a device wafer
with a plurality of electrically operative devices. The device
wafer may be divided in accordance with aspects of the present
disclosure to yield a thinner device wafer and a second thinner
wafer on which one or more epitaxial layers (optionally in
conjunction with one or more metal contacts) may be subsequently
formed. In certain embodiments, a substrate may comprise a diameter
of 150 mm or greater, or 200 mm or greater. In certain embodiments,
a substrate may comprise 4H-SiC with a diameter of 150 mm, 200 mm,
or greater, and a thickness in a range of 100 to 1000 microns, or
in a range of 100 to 800 microns, or in a range of 100 to 600
microns, or in a range of 150 to 500 microns, or in a range of 150
to 400 microns, or in a range of 200 to 500 microns, or in any
other thickness range or having any other thickness value specified
herein.
[0192] Various embodiments refer to laser subsurface damage
including lines that are oriented relative to a crystal structure
of a substrate. In certain embodiments, a substrate comprises a
crystalline material having a hexagonal crystal structure, wherein
laser damage lines are oriented perpendicular to, or within .+-.5
degrees of perpendicular to, a <1120> direction of the
hexagonal crystal structure and parallel or substantially parallel
to (e.g., within .+-.5 degrees, .+-.3 degrees, or .+-.1 degree of)
a surface of the substrate. Although a primary flat on a
conventional 4H-SiC wafer is intended to be oriented parallel to
the <1120> direction of the hexagonal crystal structure, a
primary flat may not be truly parallel to such direction due to
variations in manufacturing. Various SiC wafer manufacturers
provide published specifications for primary flat orientation of
.+-.5 degrees from parallel to the <1120> direction of the
hexagonal crystal structure. It is therefore preferred to use x-ray
diffraction (XRD) data rather than wafer flat alignment to
determine proper laser orientation for formation of subsurface
laser damage.
[0193] Methods disclosed herein may be applied to substrates of
various crystalline materials, of both single crystal and
polycrystalline varieties. In certain embodiments, methods
disclosed herein may utilize cubic, hexagonal, and other crystal
structures, and may be directed to crystalline materials having
on-axis and off-axis crystallographic orientations. In certain
embodiments, methods disclosed herein may be applied to
semiconductor materials and/or wide bandgap materials. Exemplary
materials include, but are not limited to, Si, GaAs, and diamond.
In certain embodiments, such methods may utilize single crystal
semiconductor materials having hexagonal crystal structure, such as
4H-SiC, 6H-SIC, or Group III nitride materials (e.g., GaN, AlN,
InN, InGaN, AlGaN, or AlInGaN). Various illustrative embodiments
described hereinafter mention SiC generally or 4H-SiC specifically,
but it is to be appreciated that any suitable crystalline material
may be used. Among the various SiC polytypes, the 4H-SiC polytype
is particularly attractive for power electronic devices due to its
high thermal conductivity, wide bandgap, and isotropic electron
mobility. Bulk SiC may be grown on-axis (i.e., with no intentional
angular deviation from the c-plane thereof, suitable for forming
undoped or semi-insulating material) or off-axis (typically
departing from a grown axis such as the c-axis by a non-zero angle,
typically in a range of from 0.5 to 10 degrees (or a subrange
thereof such as 2 to 6 degrees or another subrange), as may be
suitable for forming N-doped or highly conductive material).
Embodiments disclosed herein may apply to on-axis and off-axis
crystalline materials, as well as doped and unintentionally doped
crystalline semiconductor materials. Doped semiconductor material
(e.g., N-doped SiC) exhibits some infrared absorption, thus
requiring the use of higher laser power than undoped material to
impart subsurface laser damage. In certain embodiments, crystalline
material may include single crystal material, and may further
include single crystal semiconductor material. Certain embodiments
disclosed herein may utilize on-axis 4H-SiC or vicinal (off-axis)
4H-SiC having an offcut in a range of from 1 to 10 degrees, or from
2 to 6 degrees, or about 4 degrees.
[0194] FIGS. 6A and 6C schematically illustrate on-axis and
off-axis crystalline substrates in the form of ingots that may be
utilized with methods disclosed herein. FIG. 6A is a side elevation
schematic view of an on-axis ingot 15 of crystalline material
having first and second end faces 16, 17 that are perpendicular to
the c-direction (i.e., [0001] direction for a hexagonal crystal
structure material such as 4H-SiC). FIG. 6B is a side elevation
schematic view of the ingot 15 of FIG. 6A being rotated by four
degrees, with a superimposed pattern 18 (shown in dashed lines) for
cutting and removing end portions of the ingot 15 proximate to the
end faces 16, 17. FIG. 6C is a side elevation schematic view of an
off-axis ingot 15A formed from the ingot 15 of FIG. 6B, following
removal of end portions to provide new end faces 16A, 17A that are
non-perpendicular to the c-direction. If laser emissions of a first
depth are supplied through an end face 16 of the ingot 15 to form
subsurface laser damage, a carrier (not shown) is joined to the end
face 16, and the ingot 15 is fractured along the subsurface laser
damage, then an on-axis wafer may be formed. Conversely, if laser
emissions of a first depth are supplied through an end face 16A of
the off-axis ingot 15A to form subsurface laser damage, a carrier
(not shown) is joined to the end face 16A, and the ingot 15A is
fractured along the subsurface laser damage, then an off-axis wafer
may be formed.
Subsurface Laser Damage Formation
[0195] Tools for forming laser subsurface damage in crystalline
materials are known in the art and commercially available from
various providers, such as Disco Corporation (Tokyo, Japan). Such
tools permit laser emissions to be focused within an interior of a
crystalline material substrate, and enable lateral movement of a
laser relative to the substrate. Typical laser damage patterns in
the art include formation of parallel lines that are laterally
spaced relative to one another at a depth within a crystalline
substrate. Parameters such as focusing depth, laser power,
translation speed, and subsurface damage line spacing may be
adjusted to impart laser damage, but adjustment of certain factors
involves tradeoffs. Increasing laser power tends to impart greater
subsurface damage that may enhance ease of fracturing (e.g., by
reducing the stress required to complete fracturing), but greater
subsurface damage increases surface irregularities along surfaces
exposed by fracturing, such that additional processing may be
required to render such surfaces sufficiently smooth for subsequent
processing (e.g., for incorporation in electronic devices), and the
additional processing leads to additional kerf losses. Reducing
lateral spacing between subsurface laser damage lines may also
enhance ease of fracturing, but a reduction in spacing between
laser damage lines increases the number of translational passes
between a substrate and a laser, thereby reducing tool
throughput.
[0196] FIG. 7 is a perspective view schematic of one example of a
laser tool 29 configured to focus laser emissions within an
interior of a crystalline material 30 to form subsurface damage 40.
The crystalline material 30 includes an upper surface 32 and an
opposing lower surface 34, and the subsurface damage 40 is formed
in the interior of the crystalline material 30 between the upper
and lower surfaces 32, 34. Laser emissions 36 are focused with a
lens assembly 35 to yield a focused beam 38, with a focal point
thereof being in the interior of the crystalline material 30. Such
laser emissions 36 may be pulsed at any suitable frequency
(typically in the nanosecond, picosecond, or femtosecond range) and
beam intensity, with a wavelength below the bandgap of the
crystalline material 30 to permit the laser emissions 36 to be
focused at a targeted depth below a surface thereof. At the focal
point, the beam size and short pulse width results in an energy
density high enough to result in very localized absorption that
forms subsurface damage. One or more properties of the lens
assembly 35 may be altered to adjust a focal point of the focused
beam 38 to a desired depth within the crystalline material 30.
Relative lateral motion (e.g., lateral translation) between the
lens assembly 35 and the crystalline material 30 may be effected to
propagate the subsurface damage 40 in a desired direction, as
schematically illustrated by dashed line 44. Such lateral movement
may be repeated in various patterns, including patterns as
described hereinafter.
[0197] FIGS. 8A and 8B provide exemplary laser tool travel paths
relative to a crystalline material for formation of subsurface
damage within the crystalline material. In certain embodiments, a
laser tool portion (e.g., including a lens assembly) may be
configured to move while a crystalline material is stationary; in
other embodiments, a laser tool portion may be held stationary
while a crystalline material is moved relative to the tool portion.
FIG. 8A shows a reversing y-direction linear scanning movement 46
suitable for forming subsurface damage in a pattern of laterally
spaced parallel lines within a first crystalline material 45A. FIG.
8B shows y-direction linear scanning movement 48 over (and beyond)
an entire surface of a crystalline material 45B (with slight
advancement in an x-direction upon each reversal in y-direction),
sufficient to form parallel subsurface laser damage lines
distributed through the crystalline material 45B. As shown, the
laser damage lines are perpendicular to the [1120] direction of a
hexagonal crystal structure of the crystalline material 45B along a
surface of the crystalline material 45B, and are and substantially
parallel to the surface of the crystalline material 45B.
[0198] Coverage of an entire surface of a crystalline material with
laser lines formed in a y-direction, with unidirectional
advancement in the x-direction following each y-direction reversal,
may be referred to as a single pass of laser damage formation. In
certain embodiments, laser processing of crystalline material to
form subsurface damage may be performed in two, three, four, five,
six, seven, or eight passes, or any other suitable number of
passes. Increasing the number of passes at lower laser power can
reduce kerf losses. To achieve a desirable balance of material loss
versus process speed, desirable numbers of laser subsurface damage
formation passes have been found to be two to five passes, or three
to four passes, prior to performance of a fracturing step.
[0199] In certain embodiments, lateral spacing between adjacent
laser subsurface damage lines (whether formed in a single pass or
multiple passes) may be in a range of from 80 to 400 microns, or
from 100 to 300 microns, or from 125 to 250 microns. Lateral
spacing between adjacent laser subsurface damage lines impacts
laser processing time, ease of fracture, and (depending on c-plane
orientation or mis-orientation) effective laser damage depth.
[0200] It has been observed that forming subsurface laser damage
lines in crystalline material results in formation of small cracks
in the interior of the material propagating outward (e.g.,
laterally outward) from the laser damage lines. Such cracks appear
to extend substantially or predominantly along the C-plane. The
length of such cracks appear to be functionally related to laser
power level (which may be calculated as the product of pulse
frequency times energy per pulse). For adjacent laser subsurface
damage lines spaced apart by a specific distance, it has been
observed that increasing laser power in forming such laser
subsurface damage lines tends to increase the ability of cracks to
connect or join between the laser subsurface damage lines, which is
desirable to promote ease of fracturing.
[0201] If the crystalline material subject to laser damage
formation includes an off-axis (i.e., non c-plane) orientation
(e.g., in a range of from 0.5-10 degrees, 1-5 degrees, or another
misorientation), such misorientation may affect desirable laser
damage line spacing.
[0202] A SiC substrate may include surfaces that are misaligned
(e.g., off-axis at an oblique angle relative to) the c-plane. An
off-axis substrate may also be referred to as a vicinal substrate.
After fracturing such a substrate, the as-fractured surface may
include terraces and steps (which may be smoothed thereafter by
surface processing such as grinding and polishing). FIG. 9 is a
perspective view schematic of a surface structure of an off-axis
4H-SiC crystal 50 (having an angle A relative to a c-axis basal
plane) after fracture but prior to smoothing. The fractured surface
exhibits steps 52 and terraces 54 relative to a c-axis basal plane
56. For a 4 degree off-axis surface, steps theoretically have a
height of about 17 microns for a terrace width of 250 microns. For
a 4H-SiC crystal having subsurface laser damage, 250 micron spacing
between laser lines forms terraces of 250 micron width. After
fracturing, the stepped surface is subject to being ground smooth,
planarized, and polished in preparation for epitaxial growth of one
or more layers thereon.
[0203] When subsurface laser damage is formed in crystalline
material (e.g., SiC), and if subsurface laser damage lines are
oriented away from perpendicular to a substrate flat (i.e.,
non-perpendicular to the [1120] direction), then such laser damage
lines extend through multiple steps and terraces in a manner
equivalent to off-axis semiconductor material. For purposes of
subsequent discussion, the term "off-axis laser subsurface damage
lines" will be used to refer to laser subsurface damage lines that
are non-perpendicular to the [1120] direction.
[0204] Providing spacing that is too large between adjacent
subsurface laser damage lines inhibits fracture of crystalline
material. Providing spacing that is too small between adjacent
subsurface laser damage lines tends to reduce step heights, but
increases the number of vertical steps, and increasing the number
of vertical steps typically requires greater separation force to
complete fracturing.
[0205] Reducing spacing between adjacent laser damage lines to a
distance that is too small may yield diminishing returns and
substantially increase processing time and cost. A minimum laser
energy threshold is required for SiC decomposition. If this minimum
energy level creates connected cracks between two laser lines
spaced about 100 microns apart, then reducing laser line spacing
below this threshold likely offers little benefit in terms of
reducing kerf loss.
[0206] Surface roughness of crystalline material exposed by
fracturing can impact not only subsequent handling such as robot
vacuum, but also grind wheel wear, which is a primary consumable
expense. Roughness is impacted by both the spacing of subsurface
laser damage lines and orientation of such subsurface damage lines
relative to the crystal structure of the semiconductor material.
Reducing a gap between subsurface damage lines simply reduces
potential step height. Providing off-axis laser subsurface damage
lines tends to breaks up the long parallel steps that would
otherwise be present at the laser damage region, and it also helps
mitigate at least some impact from C plane slope or curvature. When
the laser lines are perpendicular to the flat of a substrate, the
cleave plane parallel to the laser lines along the C plane extends
about 150 mm from the flat to the opposing curved end of the wafer.
Slight deviations in the C plane slope or curvature (which are
common for SiC substrates) can create significant variability in
the fractured surface as it forces plane jumping as a fracture
propagates. A drawback to providing off-axis laser subsurface
damage lines is that such subsurface damage lines generally require
laser power to be increased to form connected cracks between
adjacent laser lines. Thus, in certain embodiments, forming a
combination of on-axis subsurface laser damage lines (that are
perpendicular to the primary flat) and off-axis laser subsurface
damage lines provides a good balance between avoiding excessive
variability in the fractured surface without requiring unduly
increased laser power to form connected cracks between adjacent
laser lines.
[0207] In certain embodiments, a laser having a wavelength of 1064
nm may be used to implement methods disclosed herein, with the
inventors having gained experience in processing of 4H-SiC.
Although a wide range of pulse frequencies may be used in certain
embodiments, pulse frequencies of 120 kHz to 150 kHz have been
successfully employed. A translation stage speed of 936 mm/s
between a laser and a substrate to be processed has been
successfully utilized; however, higher or lower translation stage
speeds may be used in certain embodiments with suitable adjustment
of laser frequency to maintain desirable laser pulse overlap.
Average laser power ranges for forming subsurface laser damage in
doped SiC material are in a range of from 3 W to 8 W, and 1 W to 4
W for undoped SiC material. Laser pulse energy may be calculated as
power divided by frequency. Laser pulse widths of 3 ns to 4 ns may
be used, although other pulse widths may be used in other
embodiments. In certain embodiments, a laser lens Numerical
Aperture (NA) in a range of 0.3 to 0.8 may be used. For embodiments
directed to processing of SiC, given the refractive index change
going from air (.about.1) to SiC (.about.2.6), a significant change
in refractive angle is experience inside SiC material to be
processed, making laser lens NA and aberration correction important
to achieving desirable results.
[0208] One of the primary drivers of kerf loss is subsurface laser
damage below the primary fracture region on the ingot side. In
general, an increase in subsurface laser damage increases kerf
loss. One potential cause of increased subsurface laser damage is a
failure to adequately compensate for the optical characteristics of
the crystalline material. In certain embodiments, optical parameter
optimization may be periodically performed (e.g., each time a
crystalline material substrate (e.g., ingot) is supplied to the
laser tool) prior to formation of subsurface laser damage in a
substrate. Such optimization may utilize variable height adjustment
to attainment of an initial state in which a best focus point of
the laser beam is formed an upper surface of the crystalline
material substrate, followed by adjusting the aperture and/or
correction collar adjustment ring of the laser tool corresponding
to a desired depth of formation of subsurface laser damage in the
crystalline material according to a subsequent state.
[0209] In certain embodiments, a crystalline material substrate may
exhibit doping that varies with respect to position (e.g.,
laterally and/or with diameter) across a primary surface (e.g.,
face) of the substrate. For example, growth of doped SiC may result
in formation of a doping ring region (as shown in FIGS. 14A, 14C,
and 14D) of increased dopant concentration, resulting in higher
laser absorption and slightly altered refractive index, wherein
both of the foregoing impact depth of focusing of laser emissions
in the substrate. Increasing laser power when impinging focused
laser emissions into the doping ring region, relative to power used
when impinging focused laser emissions into the material outside
the doping ring region, can compensate for differing properties of
the doping ring region. In certain embodiments, presence of a
condition indicative of non-uniform doping of a crystalline
material across at least a portion of a surface of the substrate
may be detected to determine presence of at least one first doping
region and at least one second doping region. (Methods for
detecting different doping conditions include, but are not limited
to, interferometry, resistivity measurement, absorption or
reflectivity measurement, and other techniques known to those
skilled in the art.) Thereafter, responsive to detection of the
condition indicative of non-uniform doping of the crystalline
material, laser power may be altered during formation of subsurface
laser damage patterns to provide laser emissions at a first average
power when forming subsurface laser damage in a first doping
region, and to provide laser emissions at a second average power
when forming subsurface laser damage in a second doping region,
wherein the first and second average power levels differ from one
another.
[0210] In certain embodiments, a crystalline material substrate may
exhibit laser absorption levels that vary with respect to vertical
position in the substrate (e.g., within an ingot), particularly for
intentionally doped material. Laser absorption levels may also vary
from substrate to substrate (e.g., from ingot to ingot). It is
believed that such changes may be attributable to doping changes.
In certain embodiments, a lower average laser power (e.g., 3 W) may
be used for formation of subsurface laser damage in a substrate
region distal from a growth seed, and a higher average laser power
(e.g., 5.5 W) may be used for formation of subsurface laser damage
in a substrate region proximal to a growth seed.
[0211] In certain embodiments, for initial setting of laser
subsurface damage to the correct depth relative to a surface of
crystalline material substrate, an optical measurement of the depth
of laser focus in the semiconductor material may be performed
(e.g., taking into account semiconductor material/air index of
refraction changes), and the setting of laser damage (e.g., laser
power and/or laser focus) may be adjusted responsive to such
measurement prior to scanning an entire surface of the substrate.
In certain embodiments, an optical measurement of a depth of laser
focus may be performed once per ingot, or each time after a portion
of an ingot is fractured and removed (i.e., before formation of
subsurface laser damage pattern(s) for each substrate layer to be
removed by subsequent fracturing).
[0212] In certain embodiments, a semiconductor material processing
method as disclosed herein may include some or all of the following
items and/or steps. A second carrier wafer may be attached to a
bottom side of a crystalline material substrate (e.g., ingot).
Thereafter, a top side of the crystalline material substrate may be
ground or polished, such as to provide an average surface roughness
R.sub.a of less than about 5 nanometers to prepare the surface for
transmitting laser energy. Laser damage may then be imparted at a
desired depth or depths within the crystalline material substrate,
with spacing and direction of laser damage traces optionally being
dependent on crystal orientation of the crystalline material
substrate. A first carrier may be bonded to a top side of the
crystalline material substrate. An identification code or other
information linked to the first carrier is associated with a wafer
to be derived from the crystalline material substrate.
Alternatively, laser marking may be applied to the wafer (not the
carrier) prior to separation to facilitate traceability of the
wafer during and after fabrication. The crystalline material
substrate is then fractured (using one or more methods disclosed
herein) along a subsurface laser damage region to provide a portion
of the semiconductor material substrate bound to the first carrier,
and a remainder of the crystalline material substrate being bound
to the second carrier. Both the removed portion of the
semiconductor material substrate and the remainder of the
semiconductor material substrate are ground smooth and cleaned as
necessary to remove residual subsurface laser damage. The removed
portion of the semiconductor material substrate may be separated
from the carrier. Thereafter, the process may be repeated using the
remainder of the semiconductor material substrate.
[0213] Whereas wire sawing of SiC wafers typically entails kerf
losses of at least about 250 microns per wafer, laser- and
carrier-assisted separation methods disclosed herein and applied to
SiC may achieve kerf losses in a range of from 80 to 140 microns
per wafer.
[0214] In certain embodiments, laser subsurface damage may be
formed in a crystalline material substrate prior to bonding the
substrate to a rigid carrier. In certain embodiments, a rigid
carrier that is transparent to laser emissions of a desired
wavelength may be bonded to a crystalline material substrate prior
to subsurface laser damage formation. In such an embodiment, laser
emissions may optionally be transmitted through a rigid carrier and
into an interior of the crystalline material substrate. Different
carrier-substrate subsurface laser formation configurations are
shown in FIGS. 10A-10D. FIG. 10A is a schematic view of laser
emissions 61 being focused through a surface of a bare substrate 62
to form subsurface laser damage 63 within the substrate 62, whereby
a rigid carrier may be affixed to the substrate 62 following
formation of the subsurface laser damage. FIG. 10B is a schematic
view of laser emissions 61 being focused through a surface of a
substrate 62 form subsurface laser damage 63 within the substrate
62, with the substrate 62 having previously been bonded using
adhesive material 64 to a rigid carrier 66. FIG. 10C is a schematic
view of laser emissions 61 being focused through a rigid carrier 66
and adhesive 64 to form subsurface laser damage 63 within a
substrate 62 previously bonded to the rigid carrier 66. In certain
embodiments, a surface of the substrate 62 distal from the rigid
carrier 66 may include one or more epitaxial layers and/or
metallization layers, with the substrate 62 embodying an operative
electrical device prior to formation of the subsurface laser damage
63. FIG. 10D is a schematic view of laser emissions 61 being
focused through a rigid carrier 66 into a substrate 62 (without an
intervening adhesive layer) to form subsurface laser damage 63
within the substrate 62 previously bonded (e.g., via anodic bonding
or other adhesiveless means) to the rigid carrier 66.
Interspersed Subsurface Laser Damage
[0215] In certain embodiments, subsurface laser damage may be
formed in crystalline material by sequential formation of multiple
interspersed laser damage patterns, with each subsurface laser
damage pattern including a plurality of substantially parallel
lines. In certain embodiments, each subsurface laser damage pattern
may extend over substantially an entire length (e.g., perpendicular
to a substrate flat) and include spaced-apart lines distributed
over substantially an entire width, of a substrate of crystalline
material. In certain embodiments, interspersed damage patterns may
include sequentially formed first and second, or first through
third, or first through fourth, subsurface laser damage patterns,
with each subsurface laser damage pattern including multiple
parallel lines. It is believed that sequentially forming multiple
subsurface laser damage patterns in an interspersed fashion (e.g.,
forming a first subsurface damage pattern, then forming a second
subsurface damage pattern, then forming any subsequent subsurface
damage pattern(s), with various lines of each damage pattern
distributed among the other damage patterns) is preferable to
forming the same traces without interspersing to promote ease of
fracturing of the crystalline material along or adjacent to a
subsurface laser damage region. Without wishing to be bound by any
specific theory as to reasons for improved fracturing results
obtained by interspersing of subsurface laser damage patterns in a
crystalline material, it is believed that sequential formation of
interspersed subsurface laser damage patterns may preserve a
greater degree of internal stress within the semiconductor material
to facilitate lateral propagation of cracks emanating from
different subsurface laser damage lines.
[0216] In certain embodiments, a first subsurface laser damage
pattern in a crystalline material includes a first plurality of
parallel lines and a first plurality of cracks in the interior of
the crystalline material propagating laterally outward (e.g.,
predominantly or substantially along the c-plane) from lines of the
first plurality of substantially parallel lines, wherein cracks
emanating from each line are non-connecting with cracks emanating
from each adjacent line. In certain embodiments, a second
subsurface laser damage pattern including a second plurality of
parallel lines is formed in the crystalline material after
formation of the first subsurface laser damage pattern, wherein the
second subsurface laser damage pattern includes a second plurality
of cracks in the interior of the crystalline material propagating
laterally outward from lines of the second plurality of
substantially parallel lines, and at least some cracks of the
second plurality of cracks connect with cracks emanating from two
adjacent lines of the first plurality of lines (e.g., to form
continuous cracks).
[0217] In certain embodiments, first, second, and third subsurface
laser damage patterns are sequentially formed in a crystalline
material, with each subsurface laser damage pattern including
multiple parallel lines, and with lines of each subsurface laser
damage pattern being distributed among lines of each other
subsurface laser damage pattern. In certain embodiments, the first
subsurface laser damage pattern comprises a first plurality of
cracks in the interior of the crystalline material propagating
laterally outward from lines of the first plurality of
substantially parallel lines; the second subsurface laser damage
pattern comprises a second plurality of cracks in the interior of
the crystalline material propagating laterally outward from lines
of the second plurality of substantially parallel lines, with the
second plurality of cracks being non-connecting with the first
plurality of cracks; and the third subsurface laser damage pattern
comprises a third plurality of cracks in the interior of the
crystalline material propagating laterally outward from lines of
the third plurality of substantially parallel lines. In such an
embodiment, at least some cracks of the third plurality of cracks
connect with (i) at least some cracks of the first plurality of
cracks and (ii) at least some cracks of the second plurality of
cracks (e.g., to form continuous cracks). In certain embodiments, a
fourth subsurface laser damage pattern may be formed after the
first through third subsurface laser damage patterns, with the
fourth subsurface laser damage pattern serving to further connect
cracks emanating from any two or more of the first, second, or
third lines. In certain embodiments, three, four, five, or more
interspersed patterns of subsurface laser damage may be
provided.
[0218] In certain embodiments, one or more portions of a substrate
may include interspersed subsurface laser damage patterns, while
other portions of a substrate may include non-interspersed laser
damage patters. In certain embodiments, different interspersing
patterns of subsurface laser damage may be provided on the same
substrate. For example, an interspersing pattern of subsurface
laser damage on a single substrate may include five damage lines in
a first region, four damage lines in a second region, three damage
lines in a third region, two damage lines in a fourth region, one
damage lines in a fifth region (i.e., without interspersing), zero
damage patterns in a sixth region, or any combination of two or
three of the foregoing, optionally wherein each of the foregoing
regions has substantially the same unit area. In certain
embodiments, a regular (e.g., regularly repeating) pattern of
interspersed damage lines may exist in at least one region of the
substrate, and an irregular (e.g., lacking regular repeat) pattern
of interspersed damage lines or non-interspersed damage lines may
exist in at least one other region of the substrate.
[0219] FIG. 11A provides a top plan view of a crystalline material
substrate 70 including interspersed first, second, and third
subsurface laser damage patterns defined therein according to one
embodiment. The first, second, and third subsurface damage patterns
separately include first, second, and third pluralities of parallel
lines 71, 72, 73, respectively, that extend perpendicular to a
primary substrate flat 78 (and perpendicular to the [1120]
direction). The three laser damage patterns in combination forming
multiple three-line groups 74 that are separated from one another
by an inter-group spacing 75 that exceeds a spacing 76, 77 between
adjacent lines in each three-line group 74. For clarity, cracks
formed by the first, second, and third pluralities of parallel
lines 71, 72, 73 are not illustrated in FIG. 11A. In certain
embodiments, the first plurality of parallel lines 71 is formed in
a first pass, the second plurality of parallel lines 72 is formed
in a second pass, and the third plurality of parallel lines 73 is
formed in a third pass. The third pass may serve to connect cracks
originally emanating from any of the first and/or second parallel
lines 71, 72.
[0220] With continued reference to FIG. 11A, in one embodiment the
first plurality of parallel lines 71 may be formed with a 500
micron pitch (i.e., spacing between lines), and the second
plurality of parallel lines 72 may be formed with a 500 micron
pitch and an offset of 250 microns relative to the first plurality
of parallel lines 71. Thereafter, the third plurality of parallel
lines 73 may be formed with a 500 micron pitch, and an offset of
125 microns relative to the first plurality of parallel lines 71.
This arrangement creates multiple three-line groups 74 that are
separated from each other three-line group by a 250 micron gap,
with adjacent lines within each three-line group being separated
from each other by a gap of 125 microns.
[0221] The inventors have found that the order of the three-pass
laser damage formation process described in connection with FIG.
11A is important. If the order of passes is changed to sequentially
form the first, third, and second pluralities of subsurface laser
damage lines, then higher laser power is required to complete
cracking across the 250 micron inter-group spacing 75. It is
believed that this may be attributable to the cracking that occurs
between the 125 micron spaced lines on the second pass when using
the original (first, second, third pass) sequential order, in which
cracks formed in the third pass are sufficiently sized to just
connect cracks emanating from the second subsurface damage lines
across the second 125 micron gap 77. When the order of the passes
is first, third, second, then cracking across the inter-group
spacing 75 is not observed unless laser power is increased, but
laser power increases typically increase kerf loss. Thus, according
to certain embodiments in which the order of passes is first,
second, third, it may be desirable for cracks formed in the first
and second passes not to connect with one another, and then for
cracks formed in the third pass to create connected cracks across
both the 125 micron gaps 76, 77 and the 250 micron inter-group
spacing 75.
[0222] In certain embodiments, boundaries of each three-line group
74 may be considered to bound a damage-bearing area of the
substrate 70, and the damage-bearing area of each three-line group
74 is spaced apart from a damage-bearing area of each other
three-line group (i.e., by the inter-group spacing 75). Notably, as
will be shown in FIG. 11D, the cracks formed by subsurface laser
damage may propagate between adjacent three-line groups 74 across
the inter-group spacing 75.
[0223] FIGS. 11B-11D illustrate fabrication of the crystalline
material substrate 70 of FIG. 11A. FIG. 11B illustrates the
substrate 70 following formation of a first plurality of subsurface
laser damage lines 71 (perpendicular to a flat 78 of the substrate
70) having a pitch (or inter-line spacing) 71B and that form a
first subsurface laser damage pattern 71A. Cracks 71C propagate
laterally outward from the first plurality of subsurface laser
damage lines 71, but cracks emanating from different subsurface
laser damage lines 71 do not connect with one another.
[0224] FIG. 11B illustrates the substrate 70 following formation of
a second plurality of subsurface laser damage lines 72
(perpendicular to a flat 78 of the substrate 70) having a pitch (or
inter-line spacing) 72B and that form a second subsurface laser
damage pattern 72A. Cracks 72C propagate laterally outward from the
second plurality of subsurface laser damage lines 71, but cracks
emanating from different subsurface laser damage lines 71 do not
connect with one another.
[0225] FIG. 11C illustrates the substrate 70 following formation of
a third plurality of subsurface laser damage lines 73
(perpendicular to a flat 78 of the substrate 70) having a pitch (or
inter-line spacing) 73B and that form a third subsurface laser
damage pattern 73A. Cracks 73C propagate laterally outward from the
third plurality of subsurface laser damage lines 73, with such
cracks 73C being sufficient to connect cracks 71C, 72C formed by
the first and second subsurface laser damage lines 71, 72. As
shown, connection of cracks between the first, second, and third
pluralities of subsurface damage lines is also sufficient to cause
cracks to further propagate and connect across the inter-group
spacing 75.
[0226] In certain embodiments, a third laser pass that forms the
third subsurface damage pattern is performed at a higher laser
power level than the first two passes, to assist in extending
cracks to connect across the inter-group spacing 75, which is wider
than the spacing 76, 77 between lines within each three-line group
74. The inventors have found that increasing laser power during the
third pass sufficient to connect not only cracks between laser
subsurface damage line 125um apart, but also between laser
subsurface damage lines positioned 250 um apart (such as shown in
FIG. 11D). This yields a roughly 25% tool throughput increase with
a small penalty in kerf loss (e.g., approximately 110 um kerf loss
instead of 100 um).
[0227] In certain embodiments, all laser subsurface damage lines
may be non-perpendicular to a primary substrate flat (and to the
[1120] direction), within a range of from about 1 degree to 5
degrees from perpendicular. For example, FIG. 12 is top plan
schematic view of a crystalline material substrate 80 including a
substrate flat 88 and first, second, and third pluralities of
substantially parallel subsurface laser damage lines 81-83 that are
interspersed or interspersed among one another to form first
through third subsurface laser damage patterns. Each plurality of
substantially parallel subsurface laser damage lines 81-83 deviates
three degrees relative to perpendicular to a primary substrate flat
(and to the [1120] direction), with the laser damage patterns in
combination forming multiple three-line groups 89 that are
separated from one another by an inter-group spacing 85 that
exceeds the spacing (or gaps) 86, 87 between adjacent lines in each
three-line group 89. In one embodiment the first plurality of
parallel lines 81 may be formed with a 500 micron pitch (i.e.,
spacing between lines), and the second plurality of parallel lines
82 may be formed with a 500 micron pitch and an offset of 250
microns relative to the first plurality of parallel lines 81.
Thereafter, the third plurality of parallel lines 83 may be formed
with a 500 micron pitch, and an offset of 125 microns relative to
the first plurality of parallel lines 81. This arrangement creates
multiple three-line groups 89 that are separated from each other
three-line group by a 250 micron gap, with adjacent lines within
each three-line group being separated from each other by a gap of
125 microns. As shown, the parallel subsurface laser damage lines
81-83 of each group are parallel to one another.
[0228] FIG. 13 is top plan schematic view of a crystalline material
substrate 90 including a substrate flat 98 and first through fourth
pluralities of substantially parallel subsurface laser damage lines
91-94 that are interspersed or interleaved among one another to
form first through fourth subsurface laser damage patterns with all
lines being parallel to one another and perpendicular to the
substrate flat 98 (and to the [1120] direction). In certain
embodiments, the first through fourth pluralities of subsurface
laser damage lines 91-94 may each include lines having a pitch of
500 nm, wherein the second plurality of lines 92 are offset 250
microns from the first plurality of lines 91, the third plurality
of lines is offset 125 microns from the first plurality of lines
91, and the fourth plurality of lines is offset 375 microns from
the first plurality of lines 91. The net result is that a 125
micron gap is provided between each line of the first through
fourth pluralities of lines 91-94. A four-line repeat group 95 is
composed of the first through fourth lines 91-94.
[0229] An alternative method for forming a crystalline material
substrate similar to the substrate 90 shown in FIG. 13 involves use
of four passes of laser subsurface damage formation, with each pass
forming lines having a 500 micron pitch. Following a first pass,
lines formed by a second pass are offset 125 microns from lines of
the first pass, then lines of the third pass are offset 250 microns
from lines of the first pass, then lines of the fourth pass are
offset 375 microns from lines of the first pass.
[0230] FIG. 14 is top plan schematic view of a crystalline material
substrate 100 including a substrate flat 108, and including
interspersed first through third pluralities of subsurface laser
damage lines 101-103 forming first, second, and third subsurface
laser damage patterns. The first and second pluralities of lines
101, 102 are each parallel to one another and perpendicular to the
primary substrate flat 108 (and to the [1120] direction), while the
third plurality of lines 103 is non-parallel to the first and
second pluralities of lines 101, 102 (e.g., with an angular
difference in a range of from 1 to 5 degrees) but does not cross
any of the first and second lines 101, 102 within the substrate
100. In certain embodiments, the first and second pluralities of
parallel lines 101, 102 are formed first, and then the third
plurality of parallel lines 103 is formed thereafter. In certain
embodiments, the first and second pluralities of parallel lines
101, 102 each have a 500 micron pitch, with the second plurality of
parallel lines 102 being offset 250 microns relative to the first
plurality of parallel lines 101. A multi-line repeat group 104 is
composed of the first through third lines 101-103.
[0231] Although subsurface laser damage lines in FIG. 14 are
non-crossing, in certain embodiments one or more subsurface laser
damage lines (e.g., formed in a subsequent laser damage formation
pass) may cross one or more other subsurface damage lines (e.g.,
formed in a prior or initial laser damage formation pass). In
certain embodiments, relative angles between subsurface laser
damage lines that cross may be in a range of 4 to 30 degrees, or 5
to 20 degrees, or 5 to 15 degrees, or 5 to 10 degrees.
[0232] FIG. 15 is top plan schematic view of a crystalline material
substrate 110 including a substrate flat 118, and including
interspersed first through third pluralities of subsurface laser
damage lines 111-113 that form first through third subsurface laser
damage patterns. The first and second pluralities of lines 111, 112
are each parallel to one another and non-perpendicular to the
primary substrate flat 108 (e.g., with an angular difference in a
range of from 1 to 5 degrees), while the third plurality of lines
113 is perpendicular to the primary substrate flat 118 but at least
some (or all) lines thereof do not cross lines of the first and
second groups of lines 111, 112 within the substrate 110. In
certain embodiments, the first and second pluralities of parallel
lines 111, 112 each have a 510 micron pitch, with the second
plurality of parallel lines 112 being offset 250 microns relative
to the first plurality of parallel lines 111. A three-line repeat
group 114 is composed of the first through third lines 111-113.
[0233] FIG. 16 is a top plan schematic view of a crystalline
material substrate including interspersed first, second, and third
subsurface laser damage patterns defined therein according to one
embodiment in which all laser damage lines are parallel to one
another, and inter-group spacing of laser damage lines is not
uniform over at least portions of the substrate.
[0234] FIG. 17 is a top plan schematic view of a crystalline
material substrate including interspersed first, second, and third
subsurface laser damage patterns defined therein according to one
embodiment in which all laser damage lines are parallel to one
another, and laser damage lines exhibit variation in intra-group
spacing, inter-group spacing, and group composition.
[0235] FIG. 18 is a top plan schematic view of a crystalline
material substrate including sequentially formed first, second, and
third subsurface laser damage patterns defined therein according to
one embodiment in which first and second groups of laser damage
lines are parallel to one another, while a third group of laser
damage lines are non-parallel to and cross the first and second
groups of laser damage lines.
[0236] FIG. 19 is a top plan schematic view of a crystalline
material substrate including sequentially formed first, second, and
third subsurface laser damage patterns in which each group of laser
damage lines includes parallel lines, and each group of laser
damage lines is non-parallel to each other group of laser damage
lines. Although FIGS. 11A to 19 illustrate embodiments including
three or four pluralities of subsurface laser damage lines, it is
to be appreciated that any suitable number of subsurface laser
damage line groups may be provided. For example, in certain
embodiments, first and second pluralities of subsurface laser
damage lines may be interspersed in the absence of third and/or
fourth pluralities of subsurface laser damage lines. In certain
embodiments, first and second pluralities of subsurface laser
damage lines may be formed in first and second passes,
respectively, with each plurality of laser damage lines having a
250 micron pitch, and with the second plurality of laser damage
lines being offset by 125 microns relative to the first plurality
of laser damage lines.
[0237] In certain embodiments, subsurface laser damage is
distributed among multiple non-overlapping areas of crystalline
material by forming a first group of subsurface laser damage sites
in non-overlapping first and second areas of the crystalline
material, followed by formation of a second group of subsurface
laser damage sites in the first and second areas, wherein at least
some (or all) sites of the second group of subsurface laser damage
sites do not cross sites of the first group of subsurface laser
damage sites is formed in the non-overlapping areas. One or more
additional groups of subsurface laser damage sites may be formed
thereafter, and distributed among the same non-overlapping first
and second areas of the crystalline material. Although first and
second areas have been described, it is to be appreciated that any
suitable number of non-overlapping areas may be defined (e.g.,
three, four, five, six, or more areas). In certain embodiments,
such areas may not only lack any overlap, but may also be spaced
apart from one another (e.g., spaced apart laterally) in a
non-contacting relationship.
[0238] FIG. 20A is a top plan view of a crystalline material
substrate 150 illustrating non-overlapping first, second, and third
areas 150A-150C in which laser damage regions may be formed.
Although shading has been added to the first and third areas 150A,
150C for illustration purposes to emphasize boundaries between the
first through third areas 150A-150C, it is to be appreciated that
an actual crystalline material substrate 150 would typically be
uniform in color. Each area 150A-150C contacts a portion of a
primary flat 150' of the substrate 150. While three areas 150A-150C
are shown in FIGS. 20A-20D, any suitable number of areas are
contemplated, such as two, three, four, five, six, or more, and
such areas may be arranged in any suitable conformation such as in
a one-dimensional array, in a two-dimensional array, in sectors
(e.g., wedge-shaped sectors) extending from a center point,
etc.
[0239] FIG. 20B is a top plan view of the crystalline material
substrate 150 of FIG. 20A following formation of a first plurality
of subsurface laser damage regions 151 in the first through third
areas 150A-150C. As shown, the laser damage regions 151 are
provided as substantially parallel lines that are substantially
perpendicular to a primary flat 150' of the substrate 150. Multiple
laser damage regions 151 are provided in each of the first through
third areas 150A-150C. Although not illustrated in FIG. 20B, it is
to be appreciated that laterally extending cracks (such as shown in
FIG. 11B) may emanate from the laser damage regions 151, but
preferably not connect between adjacent laser damage regions 151.
In certain embodiments, subsurface laser damage regions 151 of the
plurality of subsurface laser damage regions 151 may be formed in
the first area 150A, then in the second area 150B, and finally in
the third area 150C.
[0240] FIG. 20C is a top plan view of the crystalline material
substrate 150 of FIG. 20B following formation of a second plurality
of subsurface laser damage regions 152 in the first through third
areas 150A-150C. As shown, the laser damage regions 152 of the
second plurality of subsurface laser damage regions 152 are
provided as substantially parallel lines that are substantially
perpendicular to the primary flat 150', and multiple laser damage
regions 152 are provided in each of the first through third areas
150A-150C. Additionally, each laser damage region 152 of the second
plurality of subsurface laser damage regions 152 is substantially
parallel to the first plurality of subsurface laser damage regions
151. Although not shown in FIG. 20C, it is to be appreciated that
laterally extending cracks may emanate from each laser damage
region 151, 152, but such cracks preferably do not connect between
adjacent laser damage regions 151, 152. In certain embodiments,
subsurface laser damage regions 152 of the plurality of subsurface
laser damage regions 152 may be formed in same sequence as the
first subsurface laser damage regions 151 (e.g., the subsurface
laser damage regions 152 may be formed in the first area 150A, then
in the second area 1508, and finally in the third area 150C). In
this manner, laser damage regions 152 of the second plurality of
subsurface laser damage regions 152 are interspersed among laser
damage regions 151 of the first plurality of subsurface laser
damage regions 151.
[0241] FIG. 20D is a top plan view of the crystalline material
substrate of FIG. 20C following formation of a third plurality of
subsurface laser damage regions 153 in the first through third
areas 150A-150C. As shown, the laser damage regions 153 of the
third plurality of subsurface laser damage regions 153 are provided
as substantially parallel lines that are substantially
perpendicular to the primary flat 150', and multiple laser damage
regions 153 of the third plurality of subsurface laser damage
regions 153 are provided in each of the first through third areas
150A-150C. Each laser damage region 153 of the third plurality of
subsurface laser damage regions 153 may be substantially parallel
to the first and second pluralities of subsurface laser damage
regions 151, 152. Subsurface laser damage patterns provided by the
first through third subsurface laser damage regions 151-153 form a
plurality of three-line groups 154 that are spaced apart from one
another by an inter-group spacing 154' that exceeds a spacing
between adjacent laser damage regions 151-153 in each three-line
group 154. Although not shown in FIG. 20C, it is to be appreciated
that laterally extending cracks may emanate from each laser damage
region 151-153, with the cracks extending laterally among all laser
damage region 151-153 (such as shown in FIG. 11D) to facilitate
subsequent fracture of an upper portion of the substrate 150 from a
remainder of the substrate 150. In certain embodiments, subsurface
laser damage regions 153 of the plurality of subsurface laser
damage regions 152 may be formed in same sequence as the first and
second subsurface laser damage regions 151, 152 (e.g., the
subsurface laser damage regions 153 may be formed in the first area
150A, then in the second area 1508, and finally in the third area
150C). In this manner, laser damage regions 153 of the third
plurality of subsurface laser damage regions 153 are interspersed
among laser damage regions 151, 152 of the first and second
pluralities of subsurface laser damage regions 151, 152.
Parallel Processing and/or Laser Beam Splitting
[0242] In certain embodiments, multiple regions of one substrate
may be processed simultaneously to form subsurface laser damage in
multiple substrate regions, and/or multiple substrates may be
arranged within a single tool for simultaneous or substantially
simultaneous laser processing, to enhance tool throughput. In
certain embodiments, an output beam of one laser may be split into
multiple beams using one or more beam splitters, individual beams
of the beams may either be supplied to different substrates or
different areas of a single substrate, to form subsurface laser
damage therein utilizing methods disclosed herein. In certain
embodiments, multiple lasers may be used to simultaneously supply
beams to multiple substrates or multiple areas of a single
substrate, to form subsurface laser damage therein utilizing
methods disclosed herein.
[0243] FIG. 21 is a top plan schematic view of a holder 163 of a
laser processing apparatus arranged to hold four substrates
155A-155D in which subsurface laser damage may be formed with one
or more lasers. As shown, each substrate 155A-155D includes
subsurface laser damage patterns defined therein, with such
patterns including first, second, and third pluralities of
substantially parallel lines 156-158. The three laser damage
patterns in combination forming multiple three-line groups 156 that
are separated from one another by an inter-group spacing 160 that
exceeds a spacing 161, 162 between adjacent lines in each
three-line group 159. In certain embodiments, laser damage patterns
may be formed in the first and third substrates 155A, 155C with a
first laser or a first split laser beam portion, and laser damage
patterns may be formed in the second and fourth substrates 155B,
155D with a second laser or second split laser beam portion. In
certain embodiments, the holder 163 bearing the substrates
155A-155D is configured to move (e.g., in two (x, y) lateral
directions) while one or more lasers and/or focusing optics thereof
are restrained from lateral movement (but may be subject to
vertical (z-direction) movement).
[0244] FIG. 22A is a top plan schematic view of a single substrate
164 being processed with a laser beam split into multiple portions
to simultaneously form subsurface laser damage regions according to
a first subsurface laser damage pattern in multiple areas of the
substrate 164. As shown, the substrate 164 includes multiple areas
164A-164C (e.g., resembling the areas 150A-150C depicted in FIGS.
20A-20C). An initial laser damage formation step includes impinging
two split laser beam portions to simultaneously form laser damage
regions 165' in the first and second areas 164A, 164B. The
substrate 164 may be laterally indexed relative to a laser (e.g.,
in a direction opposite the rightward arrows), and a subsequent
laser damage formation step includes impinging two split laser beam
portions to simultaneously form laser damage regions 165'' in the
first and second areas 164A, 164B. This process is repeated to form
additional laser damage regions 165''', 165'''' in the first and
second areas 164A, 164B, and eventually to cover the first, second,
and third areas 164A-164C to form a first subsurface laser damage
pattern. Thereafter, the process may be repeated to form second and
third subsurface laser damage patterns, respectively, that are
interspersed with the first subsurface laser damage pattern. The
first and second split laser beam portions may be used to form a
subsurface laser damage patterns distributed over an entirety of
the substrate 164 in half the time that the patterns could be
formed with single, undivided laser beam. FIG. 22B is a top plan
schematic view of two substrates 166A, 166B supported by a holder
168 and being processed with a laser beam split into two portions
to simultaneously form subsurface laser damage according to at
least one subsurface laser damage pattern in both substrates 166A,
166B. An initial laser damage formation step includes impinging two
split laser beam portions to simultaneously form laser damage
regions 167' in the first and second substrates 166A, 166B. The
holder bearing the substrates 166A, 166B may be laterally indexed
relative to a laser (e.g., in a direction opposite the rightward
arrows), and a subsequent laser damage formation step includes
impinging two split laser beam portions to simultaneously form
laser damage regions 167'' in the first and second substrates 166A,
1668. This process is repeated to form additional laser damage
regions 167''', 167'''' in the first and second substrates 166A,
1668, and eventually to cover the first and second substrates 166A,
1668 to form a first subsurface laser damage patterns therein.
Thereafter, the process may be repeated to form second and third
subsurface laser damage patterns in the substrates 166A, 1668,
respectively, that are interspersed with the first subsurface laser
damage pattern.
Formation of Overlapping Subsurface Laser Damage at Different
Depths
[0245] In certain embodiments, initial subsurface laser damage
centered at a first depth may be formed within an interior of a
crystalline material substrate, and additional subsurface laser
damage centered at a second depth may be formed within the interior
of the substrate, wherein the additional subsurface laser damage is
substantially registered with the initial subsurface laser damage,
and a vertical extent of at least a portion of the additional
subsurface laser damage overlaps with a vertical extent of at least
a portion of the initial laser damage. Restated, one or more
subsequent passes configured to impart laser damage at a different
depth may be added on top of one or more prior passes to provide
subsurface laser damage with an overlapping vertical extent. In
certain embodiments, addition of overlapping subsurface damage may
be performed responsive to a determination (e.g., by optical
analysis) prior to fracturing that one or more prior subsurface
laser damage formation steps was incomplete. Formation of
overlapping subsurface laser damage at different depths may be
performed in conjunction with any other method steps herein,
including (but not limited to) formation of multiple interspersed
subsurface laser damage patterns.
[0246] FIG. 23A is a cross-sectional schematic view of a
crystalline material substrate 170 including a first subsurface
laser damage pattern 173 centered at a first depth relative to a
first surface 171 of the substrate 1770, with the subsurface damage
pattern 173 produced by focused emissions of a laser 179. The first
subsurface laser damage pattern 173 has a vertical extent 174 that
remains within an interior of the substrate 170 between the first
surface 171 and an opposing second surface 172. FIG. 23B is a
cross-sectional schematic view of the substrate of FIG. 23A
following formation of a second subsurface laser damage pattern 175
centered at a second depth and registered with the first subsurface
laser damage pattern 173, wherein a vertical extent 176 of the
second damage pattern 175 overlaps with a vertical extent 174 of
the first damage pattern 173 in a damage overlap region 177. In
certain embodiments, subsequent fracturing of the crystalline
material 170 may be performed along or through the damage overlap
region 177.
Formation of Non-Overlapping Subsurface Laser Damage at Different
Depths
[0247] In certain embodiments, subsurface laser damage lines may be
formed at different depths in a substrate without being registered
with other (e.g., previously formed) subsurface laser damage lines
and/or without vertical extents of initial and subsequent laser
damage being overlapping in character. In certain embodiments, an
interspersed pattern of subsurface laser damage may include groups
of laser lines wherein different groups are focused at different
depths relative to a surface of a substrate. In certain
embodiments, a focusing depth of emissions of a laser within the
interior of the substrate differs among different groups of laser
lines (e.g., at least two different groups of first and second
groups, first through third groups, first through fourth groups,
etc.) by a distance in a range from about 2 microns to about 5
microns (i.e., about 2 .mu.m to about 5 .mu.m).
Laser Tool Calibration
[0248] One of the primary drivers of kerf loss is subsurface laser
damage below the primary fracture region on the ingot side. In
general, an increase in subsurface laser damage increases kerf
loss. One potential cause of increased subsurface laser damage is a
failure to adequately compensate for the optical characteristics of
the crystalline material.
[0249] In certain embodiments, laser calibration may be performed
each time a crystalline material substrate (e.g., ingot) is
supplied to the laser tool, prior to formation of subsurface laser
damage therein. Such calibration may utilize variable height
adjustment to attainment of an initial state in a best focus point
of the laser beam is formed an upper surface of the crystalline
material substrate, followed by adjusting the aperture or
correction collar of the laser tool corresponding to a desired
depth of formation of subsurface laser damage in the crystalline
material according to a subsequent state.
Wafer Images
[0250] FIG. 24A is a perspective view photograph of a SiC wafer 180
following separation from a carrier (i.e., the thermoplastic
glue-bonded sapphire carrier 181 shown in FIG. 24B) using a
thermally-induced fracture method described herein. Both the wafer
180 and the carrier 181 have a diameter of 150 mm. No wafer
breakage was observed following thermally induced fracture. FIG.
24C is a partially tone-reversed version of the SiC wafer
photograph of FIG. 24A to emphasize contrast between a central
doping ring 182 and an annular outer portion 183 of the SiC wafer
180. FIG. 24D shows the image of FIG. 24C annotated with a
dashed-line oval to denote a boundary between the central doping
ring 182 and the annular outer portion 183 of the SiC wafer 180.
The doping ring 182 represents a region of increased doping
relative to the annular outer portion 183 of the SiC wafer. Since
doped semiconductor material such as SiC exhibits increased
absorption of IR wavelengths, higher laser power may be beneficial
when seeking to form subsurface laser damage in the SiC wafer in
the doping ring 182 as compared to the annular outer portion 183.
In certain embodiments, presence of a condition indicative of
non-uniform doping of a crystalline material across at least a
portion of a surface of the substrate may be detected, such as by
detecting a change in light reflection or absorption by optical
means to determine presence of at least one first doping region and
at least one second doping region (e.g., the doping ring 182 and
the annular outer portion 183). Thereafter, responsive to detection
of the condition indicative of non-uniform doping of the
crystalline material, laser power may be altered during formation
of subsurface laser damage patterns to provide laser emissions at a
first average power when forming subsurface laser damage in a first
doping region (e.g., the doping ring 182), and to provide laser
emissions at a second average power when forming subsurface laser
damage in a second doping region (e.g., the annular outer portion
183), wherein the first and second average power levels differ from
one another.
[0251] FIG. 25 is a perspective view photograph of a Si face of a
SiC wafer separated from an ingot by a process involving formation
of subsurface laser damage and subsequent separation, with an inset
portion (upper right) depicting an intentionally separated fragment
of the SiC wafer including an edge depicted in subsequent scanning
electron microscope (SEM) images.
[0252] FIG. 26 is a 45 times magnification SEM image, taken at a 15
degree tilt angle, of a portion of the SiC wafer fragment of FIG.
25, with superimposed arrows showing directions of the [1100] and
[1120] crystallographic planes. Laser lines are perpendicular to
the [1120] direction spaced at about 250 microns therebetween. FIG.
27 is a 1,300 times magnification SEM image, taken at a 15 degree
tilt angle, of a portion of the SiC wafer fragment of FIG. 25. FIG.
28 is a 350 times magnification SEM image, taken at a 15 degree
tilt angle, of a portion of the SiC wafer fragment of FIG. 25. As
shown in FIG. 28, off-axis cleave planes roughly correlate with the
laser spacing, but are not consistent across the entire wafer
surface. This may be attributable at least on part to variation in
laser line position on cleave planes. In this wafer, fracture was
initiated at a polycrystalline inclusion.
[0253] FIG. 29 is a 100 times magnification SEM image taken at a 2
degree tilt angle, of a portion of the SiC wafer fragment of FIG.
25. FIG. 30 is a 1,000 times magnification SEM image taken at a 2
degree tilt angle, of a portion of the SiC wafer fragment of FIG.
25. FIGS. 29 and 30 show that laser damage is fairly shallow
compared to surface features along the fracture region. Variability
in the resulting fracture damage is visible, particularly in a
central portion of FIG. 30.
[0254] FIG. 31A is a confocal laser scanning microscopy image of a
small, central portion of the SiC wafer of FIG. 25, with
superimposed crosshairs marking positions of "trenches" formed by
laser scanning. FIG. 31B is a surface profile plot of the portion
of the SiC wafer of FIG. 31A. With reference to FIG. 31B,
variability in laser line position relative to SiC cleave planes is
observable.
[0255] FIG. 32A is a confocal laser scanning microscopy image of a
larger, top-proximate (as pictured) portion of the SiC wafer of
FIG. 35, with superimposed crosshairs marking positions of
"trenches" or lines formed by laser scanning. FIG. 32B is a surface
profile plot of the top-proximate portion of the SiC wafer of FIG.
32A. In FIG. 32B, a first pair of lines corresponding to laser
damage (represented as crosshairs within oval 200) are separated by
a depth of more than 30 microns, and a second pair of lines
corresponding to laser damage (represented as crosshairs within
oval 201) are separated by a depth of more than 20 microns. An
irregular spacing between laser lines is shown in FIGS. 32A and
32B, wherein individual lines within the first pair of lines
(within oval 200) are closer to one another, and individual lines
within the second pair of lines (within oval 201), are closer to
one another than other depicted laser damage lines.
[0256] FIG. 33A is a confocal laser scanning microscopy image of a
larger, bottom-proximate (as pictured) portion of the SiC wafer of
FIG. 25, with superimposed crosshairs marking positions of
"trenches" formed by laser scanning. FIG. 33B is a surface profile
plot of the bottom-proximate portion of the SiC wafer of FIG. 33A.
FIG. 33B shows lateral distance variation between adjacent pairs of
laser damage lines, with one pair separated by 334 microns, and
another separated by 196 microns, but a maximum depth variation of
13 microns.
Fracturing of Substrate Following Formation of Subsurface Laser
Damage
[0257] As discussed previously herein, subsurface laser damage may
be formed within a crystalline material substrate to prepare the
substrate for fracturing to remove at least one thin layer of
crystalline material (e.g., a wafer) from the substrate. Although
examples of specific fracturing techniques are described
hereinafter (e.g., cooling a CTE-mismatched carrier joined to a
substrate, impinging ultrasonic waves on a substrate, or imparting
a bending moment on a carrier mounted to substrate), it is to be
appreciated that various subsurface laser damage formation
techniques described herein may be used within any suitable
fracturing techniques, including fracturing techniques already
known to one skilled in the art.
Fracturing by Cooling Rigid Carrier with Carrier/Substrate CTE
Mismatch
[0258] FIGS. 34A-34F illustrate steps of a carrier-assisted method
for fracturing a crystalline material according to one embodiment
of the present disclosure, utilizing a rigid carrier having a
greater CTE than the crystalline material joined to the crystalline
material. FIG. 34A is side cross-sectional schematic view of a
rigid carrier 202 having a layer of adhesive material 198 joined to
a first surface 203 of the rigid carrier 202, and having a second
surface 204 that opposes the first surface 203.
[0259] FIG. 34B is a cross-sectional schematic view of an assembly
188 including the rigid carrier 202 and adhesive material 198 of
FIG. 34A joined to a crystalline material substrate 190 having a
subsurface laser damage region 196 therein. The rigid carrier 202
has a greater diameter or lateral extent than the substrate 190.
The substrate 190 includes a first surface 192 proximate to the
adhesive material 198, and includes an opposing second surface 194,
with the subsurface laser damage 196 being closer to the first
surface 192 than to the second surface 194. The adhesive material
198 extends between a first surface 192 of the crystalline
substrate 190 and the first surface 203 of the rigid carrier 202.
The adhesive material 198 may be cured according to the
requirements of a selected bonding method (e.g., thermo-compression
adhesive bonding, compression-aided UV bonding, chemically reactive
bonding, etc.). In certain embodiments, a second carrier (not
shown) may be bonded to the second surface 194 of the substrate
190, with the second carrier optionally being no wider than and/or
CTE matched with the substrate 190.
[0260] FIG. 34C is a cross-sectional schematic view of the assembly
of FIG. 34B, following positioning of the second surface 204 of the
rigid carrier 202 on a support surface 208 of cooling apparatus in
the form of a cooled chuck 206 configured to receive a cooling
liquid. Contact between the rigid carrier 202 and the cooled chuck
206 causes heat to be transferred from the rigid carrier 202 to the
cooled chuck 206. During the cooling process, the rigid carrier 202
will laterally contract to a greater extent than the crystalline
material substrate 190 due to a greater CTE of the carrier 202 than
the substrate 190, such that the carrier 202 exerting shear stress
on the substrate 190. Due to the presence of subsurface laser
damage 196 near the adhesive layer 198 that joins the rigid carrier
202 to the substrate 190, the exertion of shear stress on the
substrate 190 causes the crystalline material to fracture along or
proximate to the subsurface laser damage region 196.
[0261] In certain embodiments, the cooled chuck 206 has a smaller
diameter than a diameter of the rigid carrier 202. Although the
cooled chuck 206 may be supplied with a cooling liquid, it is not
necessary for the rigid carrier 202 to reach the liquid nitrogen
temperature (-160.degree. C.) to successfully complete
thermal-induced fracture of the crystalline material substrate 190.
Favorable separation results have been obtained for fracturing
single crystal SiC material supported by a single crystal sapphire
substrate using a cooled chuck maintained at -70.degree. C. Such
temperature can be maintained using various cooling liquids, such
as liquid methanol (which remains flowable above its freezing point
at -97.degree. C.) received from a two-phase pumped evaporative
cooling system. Favorable separation results have also been
obtained by cooling a carrier, adhesive, and a substrate in a
freezer maintained at -20.degree. C., wherein such temperature may
be maintained using a single phase evaporative cooling system. The
ability to use a single phase evaporative cooling system or a
two-phase pumped evaporative cooling system rather than liquid
nitrogen significantly reduces operating costs.
[0262] FIG. 34D is a cross-sectional schematic view of a remainder
of the crystalline material substrate 190A separated from a bonded
assembly that includes the rigid carrier 202, adhesive material
198, and a portion of the crystalline material 210 removed from the
remainder of the substrate 190A, following fracture of the
crystalline material along the subsurface laser damage region. The
remainder of the crystalline material substrate 190A is bounded by
a new first surface 193 (having residual laser damage 196A) that
opposes the second surface 194. Correspondingly, the removed
portion of crystalline material 210 is bounded by a new second
surface 212 (having residual laser damage 1968) that opposes the
first surface 192. Thereafter, the bonded assembly 215 including
the rigid carrier 202, the adhesive material 198, and the removed
portion of crystalline material 160 may be withdrawn from the
cooled chuck 206.
[0263] FIG. 34E is a cross-sectional schematic view of the bonded
assembly 215 of FIG. 34D, following withdrawal from the
liquid-cooled chuck 206. Maintaining the removed portion of
crystalline material 210 attached to the rigid carrier 202
beneficially provides mechanical support for the removed portion of
crystalline material 210 to permit one or more surface processing
steps (e.g., grinding, polishing, etc.) to be performed on the new
surface 212, to remove the residual laser damage 1968 and achieve a
desirable thickness of the crystalline material 210 (e.g., via
grinding, optionally followed by chemical mechanical planarization
and/or polishing steps). In certain embodiments, laser damage
removal and thinning may include sequential grinding/polishing
operations, and any suitable polishing and cleaning steps to
prepare the new surface 212 for subsequent operations (e.g.,
surface implantation, laser marking (e.g., along a wafer flat),
formation of epitaxial layers, metallization, etc.).
[0264] FIG. 34F is a cross-sectional schematic view of the removed
portion of the crystalline material 210 supported by an upper
surface 218 of a heated vacuum chuck 216, with the rigid carrier
202 and adhesive material 198 being laterally translated away from
the removed portion of crystalline material 212 portion following
elevated temperature softening and release of the adhesive material
198. That is, the heated vacuum chuck 216 may heat the adhesive
material 198 to a sufficient temperature to soften and/or flow,
such that upon application of an external shear stress to the
second surface 204 of the rigid carrier 202, the rigid carrier 202
is permitted to laterally translate away from the removed portion
of crystalline material 212 that is temporarily held in place by
the heated vacuum chuck 216. Thereafter, the heated vacuum chuck
216 may be deactivated, and the removed portion of crystalline
material 212 embodies a free-standing material. If desired, any
residue from the adhesive 198 may be removed and cleaned from the
first surface 203 of the rigid carrier 202, and the rigid carrier
202 optionally may be re-used for another fracturing operation. The
removed crystalline material can then be used as a growth substrate
for deposition of one or more epitaxial layers and conducting metal
layers to from a device wafer then singulated to form discrete
semiconductor devices.
Fracturing Induced by Ultrasonic Energy
[0265] Another method for effectuating fracture along a
laser-induced subsurface damage zone of a crystalline material
bonded to a rigid carrier involves application of ultrasonic energy
to the crystalline material while in the bonded state. FIG. 35 is a
cross-sectional schematic view of an assembly 188A including a
crystalline material 190A having subsurface laser damage 196A and
bonded to a rigid carrier 202A using an intervening adhesive
material 198A, with assembly 188A arranged in a liquid bath 225 of
an ultrasonic generator apparatus 220. The apparatus 220 further
includes a vessel 222 arranged in contact with an ultrasonic
generating element 224, with the vessel 222 containing the liquid
bath 225. Presence of the rigid carrier 202A may reduce or
eliminate breakage of the crystalline material 190A when subjected
to ultrasonic energy, particularly if residual stress remains
between the rigid carrier 202A and the crystalline material 190A
prior to separation (e.g., due to a CTE mismatch). Such residual
stress may reduce the amount of ultrasonic energy required to
initiate fracture of the crystalline material, thereby reducing the
likelihood of material breakage.
Fracturing Induced by Mechanical Force
[0266] In certain embodiments, fracturing of a crystalline material
bonded to a rigid carrier may be promoted by (i) application of a
mechanical force (e.g., optionally localized at one or more points)
proximate to at least one edge of the carrier. Such force may
impart a bending moment in at least a portion of the carrier, with
such bending moment being transmitted to the subsurface laser
damage region to initiate fracture. An exemplary embodiment is
shown in FIGS. 36A-36C.
[0267] FIGS. 36A-36C are cross-sectional schematic views
illustrating steps for fracturing a crystalline material substrate
236 having subsurface laser damage 233 by application of a
mechanical force proximate to one edge of a carrier 238 to which
the substrate 236 is bonded. The bonded assembly includes a
crystalline material substrate 236 having a subsurface laser damage
region 233 being bonded between rigid carriers 238, 238'. Each
rigid carrier 238, 238' includes a laterally protruding tab portion
239, 239' registered with a flat 235 of the substrate 236,
providing a local increased border region that defines a recess 231
into which a tool 219 may be inserted. FIG. 36A illustrates a state
prior to insertion of the tool 219 into the recess 191. FIG. 36B
illustrates a state following insertion of the tool 219 into the
recess, when the tool 216 is tilted upward, thereby exerting a
prying force in a direction tending to promote separation between
the rigid carriers 238, 238', thereby exerting a bending moment Mon
at least one carrier 238. In certain embodiments, the substrate 236
comprises a material (e.g., 4H-SiC) having hexagonal crystal
structure and the bending moment M is oriented within .+-.5 degrees
of perpendicular to the [1120] direction (or, equivalently, within
.+-.5 degrees of parallel to the [1-100] direction) of the
hexagonal crystal structure. FIG. 36C illustrates a state following
initial fracture of the crystalline substrate 236 along the
subsurface laser damage region 233, whereby an upper portion 236 of
the crystalline material remains bonded to the upper carrier 238,
and a lower portion 236B of the crystalline material remains bonded
to the lower carrier 238', and the upper carrier 238 is tilted
upward relative to the lower carrier 238'. Such fracture yields a
first bonded assembly 229A (including the upper carrier 238 and the
upper portion 236A of the crystalline material) separated from a
second bonded assembly 229B (including the lower carrier 238' and
the lower portion 236B of the crystalline material). In certain
embodiments, mechanical force may be applied proximate to opposing
edges of a rigid carrier to which a substrate is bonded to promote
fracture of a crystalline material having subsurface laser damage
that is bonded to the carrier.
[0268] It is noted that it is specifically contemplated to combine
two or more fracturing techniques (e.g., CTE mismatch and
ultrasonic induced fracturing; or CTE mismatch and mechanical
induced fracturing; or ultrasonic induced and mechanical induced
fracturing). In certain embodiments, liquid of an ultrasonic bath
may be cooled either before or during application of ultrasonic
energy. Amount of mechanical force that may be required to complete
fracture may be affected by CTE differential between a substrate
and a carrier. In certain embodiments, CTE differential and
mechanical force may be combined. If a CTE differential between a
carrier and substrate is small or nonexistent (i.e., matched CTE),
then more mechanical force may be required to complete fracture.
Conversely, if a CTE mismatch is large, then reduced mechanical
force or no mechanical force may be required to complete
fracture.
Device Wafer Splitting Process
[0269] In certain embodiments, a laser- and carrier-assisted
separation method may be applied to a crystalline material after
formation of at least one epitaxial layer thereon (and optionally
at least one metal layer) as part of an operative
semiconductor-based device. Such a device wafer splitting process
is particularly advantageous for the ability to increase yield (and
reduce waste) of crystalline material by significantly reducing the
need for grinding away substrate material following device
formation.
[0270] FIGS. 37A-37O are cross-sectional schematic views
illustrating steps of a device wafer splitting process, according
to which a thick wafer is fractured from a crystalline material, at
least one epitaxial layer is grown on the thick wafer, and the
thick wafer is fractured to form a first and second bonded
assemblies each including a carrier and a thin wafer divided from
the thick wafer, with the first bonded assembly including the at
least one epitaxial layer as part of an operative
semiconductor-based device.
[0271] FIG. 37A illustrates a crystalline material substrate 240
having a first surface 241 and subsurface laser damage 243 arranged
at a depth relative to the first surface. FIG. 37B illustrates the
substrate 240 of FIG. 37A following addition of adhesive material
244 over the first surface 241. FIG. 37C illustrates the items
depicted in FIG. 37B following bonding of a rigid carrier 246 to
the substrate 240 using the adhesive material 244. FIG. 37D
illustrates the items of FIG. 37D following fracturing of the
substrate 240 along the subsurface laser damage 243 (e.g., using
one or more methods disclosed herein), yielding a remainder of the
substrate 240 that is separated from a bonded assembly that
includes the carrier 246, the adhesive material 244, and a
crystalline material portion (e.g., a thick wafer) 242 removed from
the substrate 240. In certain embodiments, the thick wafer 242 may
have a thickness in a range of roughly 350 to 750 microns. Exposed
surfaces 243A, 243B of the thick wafer 242 and the remainder of the
substrate 240, respectively may exhibit surface irregularities that
may be reduced by surface processing steps such as grinding, CMP,
polishing, etc. FIG. 37E shows the thick wafer 242 following
de-bonding and removal from the carrier 246, with the thick wafer
242 including a perpendicular edge profile. Perpendicular edges of
wafers fracture readily, producing unacceptable edge chips and
particles during wafer handling. To reduce the risk of breakage, a
wafer edge may be edge ground to produce a non-perpendicular wafer
edge having a beveled or rounded edge. FIG. 37F shows the thick
wafer 242 supported between opposing upper and lower gripping
portions 248A, 248B of a turntable proximate to a rotary profile
grinding tool 249 having a concave cutting surface 249A (e.g.,
impregnated with diamond particles) configured to impart a rounded
edge profile 247 to the thick wafer 242. FIG. 37G shows the thick
wafer 242 after edge grinding (also known as edge profiling), with
the thick wafer including a rounded edge 247 providing a boundary
between first and second wafer surfaces 251, 252.
[0272] FIG. 37H shows the thick wafer 242 of FIG. 37G following
deposition of one or more epitaxial layers 253 on or over the first
surface 251 of the thick wafer 251. Due to the incompatibility of
adhesives with the high temperatures inherent to epitaxy, carrier
shown in FIG. 37D is not present. FIG. 371 shows the structure FIG.
37H, following formation of conductive (e.g., metal) contacts 254
over the epitaxial layers 253 to form at least one operative
semiconductor device, with the thick wafer 242 still having a
rounded edge 247. Conventionally, grinding would be performed on
the second surface 252 to thin the thick wafer 242 to an
appropriate thickness for the resulting device (e.g., 100 to 200
microns for a Schottky diode or MOSFET). The approach disclosed
herein reduced the need for wafer grinding, and instead utilizes
laser- and carrier-assisted separation to remove a portion of the
thick wafer so that it can be surface finished and used to
fabricate another operative semiconductor device.
[0273] The inventors have found that presence of the rounded edge
247 on the thick wafer 242 inhibits controlled formation of
subsurface laser damage proximate to the edge 247, since the
rounded profile negatively affects laser focus and depth control.
To address this issue, the rounded edge 247 of the thick wafer 242
may be removed prior to further laser processing. FIG. 37J shows
the structure of FIG. 37I being subjected to grinding with an edge
grinder 256 to grind away the rounded edge 247 and impart a
substantially perpendicular edge 255 extending between the first
and second surfaces 251, 252 of the thick wafer 242, with the
epitaxial layers 253 and contacts 254 arranged over the first
surface 251.
[0274] FIG. 37K shows the structure of FIG. 37J following addition
of temporary adhesive material 257 over the first surface 251 of
the thick wafer 242, the epitaxial layers 253, and the contacts
254, in preparation for receiving and adhering a first carrier.
FIG. 37L shows the structure of FIG. 37K following addition of a
first carrier 258 over the temporary adhesive material 257, and
following formation of subsurface laser damage 259 within the thick
wafer 242 by impingement of focused laser emissions through the
second surface 252 of the thick wafer 242. FIG. 37M shows the
structure of FIG. 37L following bonding of a rigid second carrier
260 to the second surface 252 of the thick wafer 242 proximate to
the subsurface laser damage 259. For purposes of separation, the
rigid second carrier 260 will serve as a frontside carrier intended
to remove a portion (i.e., a layer) of the thick wafer 242.
[0275] In certain embodiments, laser emissions can be applied to a
freestanding device thick wafer, and first and second carriers may
be bonded to the frontside and backside of the thick wafer at
substantially the same time. In certain embodiments, adhesive
material may be applied on carriers or the wafers for one or both
of the front and back sides.
[0276] FIG. 37N shows the items of FIG. 37M following application
of at least one fracturing process as disclosed herein to fracture
the thick wafer 242 along the subsurface laser damage 259 to yield
first and second bonded subassemblies 262A, 262B. The first bonded
subassembly 262A includes a first thin wafer portion 242A
(separated from the thick wafer 242 of FIG. 37M), the epitaxial
layers 253, the contacts 254, the temporary adhesive material, and
the first carrier 258. The second bonded subassembly 262B includes
a second thin wafer portion 242B (separated from the thick wafer
242 of FIG. 37M) and the second carrier 260. Exposed surfaces 259A,
259B of the thin wafer portions 242A, 242B may exhibit surface
irregularities due to laser damage and/or fracturing that may be
reduced by conventional surface processing steps (e.g., grinding,
CMP, and/or polishing). FIG. 37O shows an operative semiconductor
device 264 derived from the first bonded subassembly 262A by
removal of the temporary adhesive 257 and the first carrier 258.
Such figure also the second thin wafer portion 242B following
removal of the second carrier 260, to prepare the second thin wafer
portion 242B for further processing (e.g., epitaxial growth).
Exemplary Method Including Re-Use of Carrier Wafers
[0277] FIG. 38 is a flowchart schematically illustrating steps of a
method according to the present disclosure. Starting at upper left,
a laser 266 may focus laser emissions below a first surface 272 of
a thick crystalline material substrate 270 (e.g., a SiC ingot) to
produce a subsurface laser damage region 268. Thereafter, a carrier
wafer 224 may be bonded to the first surface 272 of the crystalline
material substrate 270, with the carrier wafer 274 including a
first surface 276 (proximal to the first surface 272 of the
substrate 270) and a second surface 278 that opposes the first
surface 276 of the carrier wafer 274. Such bonding between the
carrier wafer 278 and the crystalline material substrate 270 may be
performed by any method disclosed herein, such as adhesive bonding
or anodic bonding. Details concerning anodic bonding between
crystalline material substrates and carriers are disclosed in U.S.
Patent Application Publication No. 2016/0189954, with the contents
of such publication hereby being incorporated by reference herein,
for all purposes. Thereafter, a fracturing process as disclosed
herein (e.g., cooling a CTE mismatched carrier, application of
ultrasonic energy, and/or application of mechanical force) is
applied to fracture the crystalline material 270 along the
subsurface laser damage region 218, causing a crystalline material
portion 280 bound to the carrier wafer 278 to be separated from a
remainder of the crystalline material substrate 270A. A newly
exposed surface 282A of the remainder of the crystalline material
substrate 270A having residual laser damage is ground smooth and
cleaned, and returned to the beginning of the process (at upper
left in FIG. 38). Also, a newly exposed surface 284 of the removed
crystalline material 280 is ground smooth while attached to the
carrier 274. Thereafter, the carrier wafer 274 may be separated
from the removed portion of the crystalline material 280, and the
crystalline material 280 may be subject to epitaxial growth of one
or more layers to form an epitaxial device 280', while the carrier
wafer 274 is cleaned and returned to the beginning of the process
(at upper left in FIG. 38) to effectuate removal of another
relatively thin section of the crystalline material substrate
270.
[0278] FIG. 39 is a cross-sectional schematic view of a portion of
the crystalline material substrate (e.g., SiC ingot) 270 of FIG. 38
showing subsurface laser damage 268 with superimposed dashed lines
identifying an anticipated kerf loss material region 290. The
anticipated kerf loss material region 290 includes laser damage
268, plus material 284 to be mechanically removed (e.g., by
grinding and polishing) from a lower face 288 (e.g., Si-terminated
face) of the crystalline material portion 280 (e.g., SiC wafer) to
be separated from the substrate 270, plus material 286 to be
mechanically removed (e.g., by grinding and polishing) from an
upper face 282A (e.g., C-terminated) face of the remainder 270A of
the substrate 270. The lower face 288 of the crystalline material
portion 280 opposes an upper face 272 thereof. In certain
embodiments, the entire kerf loss material region may have a
thickness in a range of from 80-120 microns for SiC to provide a
substrate upper face 282A and a wafer lower face 288 sufficient for
further processing.
Material Processing with Multiple Grinding Stations/Steps
[0279] In certain embodiments, crystalline material subjected to
laser processing and fracturing may be further processed with
multiple surface grinding steps to remove subsurface damage and
edge grinding to impart a beveled or rounded edge profile, wherein
an order of grinding steps is selected and/or a protective surface
coating is employed to reduce the likelihood of imparting
additional surface damage and to render a crystalline material
wafer ready for chemical mechanical planarization. Such steps may
be performed, for example, using material processing apparatuses
according to embodiments disclosed herein, wherein an exemplary
apparatus includes a laser processing station, a fracturing
station, multiple coarse grinding stations arranged in parallel
downstream of the fracturing station, and at least one fine
grinding station arranged downstream of the coarse grinding
stations. When processing wafers cut by wire sawing, it is
commonplace to perform edge grinding prior to surface grinding or
polishing to remove wire-sawing surface damage. However, it has
been found by the inventors that edge grinding of substrate
portions (e.g., wafers) having laser damage in combination with
fracture damage, the likelihood of cracking a substrate portion is
increased. While not wishing to be bound by any specific theory as
to the reason for this phenomenon, it is believed that exposed
cleave planes resulting from surface fracturing renders the
surfaces susceptible to cracking if edge grinding is performed
prior to at least some surface processing (grinding and/or
polishing). For this reason, it has been found to be beneficial to
perform at least some surface processing (e.g., grinding and/or
polishing) prior to edge grinding.
[0280] It has been found that coarse grinding steps (i.e., to
remove laser damage and fracture damage along fractured surfaces of
a substrate portion and a bulk substrate) tend to require
significantly longer to complete than the preceding steps of laser
processing and fracturing, and significantly longer than subsequent
steps of fine grinding. For that reason, multiple coarse grinding
stations are provided in parallel to remove a bottleneck in
fabrication of multiple wafers from a bulk crystalline material
(e.g., an ingot). In certain embodiments, robotic handlers may be
arranged upstream and downstream of the multiple coarse grinding
stations to control loading and unloading of substrate portions. In
certain embodiments, a carrier bonding station may be provided
between a laser processing station and a fracturing station, and a
carrier removal station may be provided upstream (either directly
or indirectly) of an edge grinding station. A carrier may desirably
remain bonded to a substrate portion during at least some surface
grinding steps to reduce the potential for breakage, particularly
for thin substrate portions (e.g., wafers); however, the carrier is
preferably removed prior to edge grinding (or prior to coating
wafer with a protective coating preceding edge grinding).
[0281] In certain embodiments, a carrier bonding station may use
carriers pre-coated with temporary bonding media, align and press
the carrier to a substrate surface, and subject the bonding media
with the necessary conditions (e.g., heat and pressure) to
effectuate bonding between the carrier and the substrate.
Alternatively, a carrier bonding station may include a coating
station that may be used to coat the carriers or substrates on
demand.
[0282] FIG. 40 is a schematic illustration of a material processing
apparatus 300 according to one embodiment, including a laser
processing station 302, a carrier bonding station 303, a material
fracturing station 304, multiple coarse grinding stations 308A,
308B arranged in parallel, a fine grinding station 312, a carrier
removal station 313, and a CMP station 314. The laser processing
station 302 includes at least one laser, and a holder for at least
one substrate arranged to receive at least one laser beam for
formation of subsurface laser damage in a crystalline material
(e.g., an ingot). The carrier bonding station 303 is configured to
bond the crystalline material (having subsurface laser damage
therein) to at least one rigid carrier. The fracturing station 304
is arranged to receive one or more assemblies (each including a
substrate bonded to a rigid carrier) from the carrier bonding
station 303, and to fracture the at least one substrate along a
subsurface laser damage region to remove a substrate portion (which
may resemble a wafer bonded to a carrier). First and second coarse
grinding stations 308A, 308B are arranged in parallel downstream of
the fracturing station 304, with a first robotic handler 306
provided to alternately deliver substrate portions (as part of
bonded assemblies) received from the fracturing station 304 to
either the first coarse grinding station 308A or the second coarse
grinding station 3048B. Downstream of the first and second coarse
grinding stations 308A, 308B, a second robotic handler 310 is
provide to deliver coarse ground substrate portions (as part of
bonded assemblies) to a fine grinding station 312. A carrier
removal station 313 is provided downstream of the fine grinding
station 312, and serves to separate ground substrate portions from
carriers. A chemical mechanical planarization (CMP) station 314 is
arranged downstream of the carrier removal station 313 to prepare
substrate portions for further processing, such as cleaning and
epitaxial growth. The CMP station 314 functions to remove damage
remaining after fine grinding, which itself removes damage
remaining after coarse grinding. In certain embodiments, each
coarse grinding station 308A, 308B comprises at least one grinding
wheel having a grinding surface of less than 5000 grit, and the
fine grinding station 312 comprises at least one grinding wheel
having a grinding surface of at least 5000 grit. In certain
embodiments, each coarse grinding station 308A, 308B is configured
to remove a thickness of 20 microns to 100 microns of crystalline
material from a crystalline material portion (e.g., wafer), and the
fine grinding station 312 is configured to remove a thickness of 3
to 15 microns of crystalline material. In certain embodiments, each
coarse grinding station 308A, 308B and/or fine grinding station 312
may include multiple grinding substations, in which different
substations comprise grinding wheels of different grits.
[0283] An apparatus according to that of FIG. 40 may be modified to
accommodate edge grinding to impart a rounded or beveled edge
profile of a crystalline substrate portion, such as a wafer. Such
an edge profile will reduce the risk of breakage of a wafer edge.
The edge grinding may not be performed when a substrate portion is
bonded to a carrier; accordingly, a carrier removal station may be
arranged upstream (either directly or indirectly) of an edge
grinding station.
[0284] FIG. 41 illustrates a material processing apparatus 320
according to one embodiment similar to that of FIG. 40, but
incorporating an edge grinding station 332. The material processing
apparatus 320 includes a laser processing station 322, a carrier
bonding station 323, a material fracturing station 324, a first
robotic handler 326, multiple coarse grinding stations 328A, 328B
arranged in parallel, a second robotic handler 328, a carrier
removal station 331, an edge grinding station 332, a fine grinding
station 334, and a CMP station 336. An exemplary edge grinding
station 332 may be arranged to grip a wafer between upper and lower
gripping portions of a turntable arranged proximate to a rotary
grinding tool having a concave during surface (e.g., such as
illustrated in FIG. 37G). Gripping of a wafer in this manner may
undesirably impart damage to a wafer surface (e.g., a Si-terminated
surface of a SiC wafer). For this reason, the edge grinding station
332 shown in FIG. 41 is arranged upstream of the fine grinding
station 334, to permit any surface damage imparted by the edge
grinding station 332 to be removed in the fine grinding station
334. Although the fine grinding station 334 may remove a small
degree of thickness of a wafer, thereby altering a rounded or
beveled edge profile produced by the edge grinding station 332, a
sufficient degree of a rounded or beveled edge profile will remain
to inhibit fracture of a wafer edge.
[0285] The apparatus 320 according to FIG. 41 may be used to
perform a method for processing a crystalline material wafer
comprising a first surface having surface damage thereon, with the
first surface being bounded by an edge. The method comprises
grinding the first surface with at least one first grinding
apparatus to remove a first part of the surface damage; following
the grinding of the first surface with the at least one first
grinding apparatus, edge grinding the edge to form a beveled or
rounded edge profile; and following the edge grinding, grinding the
first surface with at least one second grinding apparatus to remove
a second part of the surface damage sufficient to render the first
surface suitable for further processing by chemical mechanical
planarization. In certain embodiments, the first grinding apparatus
may be embodied in the coarse grinding stations 328A, 328B, the
edge grinding may be performed by the edge grinding station 332,
and the second grinding apparatus may be embodied in the find
grinding station 312. In certain embodiments, a carrier removal
step may be performed following the grinding of the first surface
with the at least one first grinding apparatus, and prior to edge
grinding the edge to form the beveled or rounded edge profile.
[0286] In certain embodiments, a protective surface coating may be
employed to reduce the likelihood of imparting additional surface
damage during edge grinding and to render a crystalline material
wafer ready for chemical mechanical planarization. Such a surface
coating may include photoresist or any other suitable coating
material, may be applied prior to edge grinding, and may be removed
after edge grinding.
[0287] FIG. 42 is a schematic illustration of a material processing
apparatus 340 according to one embodiment similar to that of FIG.
40, but incorporating a surface coating station 354 between a fine
grinding station 352 and an edge grinding station 356, and
incorporating a coating removal station 358 between the edge
grinding station 356 and a CMP station 360. The material processing
apparatus 340 further includes a laser processing station 342, a
material fracturing station 344, a first robotic handler 346,
multiple coarse grinding stations 348A, 348B arranged in parallel,
and a second robotic handler 348 upstream of the fine grinding
station 352. The coating station 354 may be configured to apply a
protective coating (e.g., photoresist) by a method such as spin
coating, dip coating, spray coating, or the like. The protective
coating should be of sufficient thickness and robustness to absorb
any damage that may be imparted by the edge grinding station 365.
For a SiC wafer, the Si-terminated surface may be coated with the
protective coating, since the Si-terminated surface is typically
the surface on which epitaxial growth is performed. The coating
removal station 358 may be configured to strip the coating by
chemical, thermal, and/or mechanical means.
[0288] The apparatus 340 according to FIG. 42 may be used to
perform a method for processing a crystalline material wafer
comprising a first surface having surface damage thereon, with the
first surface being bounded by an edge. The method comprises
grinding the first surface with at least one first grinding
apparatus (e.g., the coarse grinding stations 348A, 348B) to remove
a first part of the surface damage; thereafter grinding the first
surface with at least one second grinding apparatus (e.g., the fine
grinding station 352) to remove a second part of the surface damage
sufficient to render the first surface suitable for further
processing by chemical mechanical planarization; thereafter forming
a protective coating on the first surface (e.g., using the surface
coating station 354); thereafter edge grinding the edge to form a
beveled or rounded edge profile (e.g., using the edge grinding
station 356); and thereafter removing the protective coating from
the first surface (e.g., using the coating removal station). The
first surface may thereafter be processed by chemical mechanical
planarization (e.g., by the CMP station 360), thereby rendering the
first surface (e.g., a Si terminated surface of the wafer) ready
for subsequent processing, such as surface cleaning and epitaxial
growth.
[0289] In certain embodiments, a gripping apparatus may be
configured for holding an ingot having end faces that are
non-perpendicular to a sidewall thereof to permit an end face to be
processed with a laser for formation of subsurface damage. In
certain embodiments, gripping effectors may conform to a sloped
sidewall having a round cross-section when viewed from above. In
certain embodiments, gripping effectors may include joints to
permit gripping effectors to conform to the sloped sidewall.
[0290] FIG. 43A is a schematic side cross-sectional view of a first
gripping apparatus 362 for holding an ingot 364 having end faces
366, 368 that are non-perpendicular to a sidewall 370 thereof,
according to one embodiment. The upper end face 366 is horizontally
arranged to receive a laser beam 376. The lower end face 368 may
have a carrier 372 attached thereto, with a chuck 374 (e.g., a
vacuum chuck) retaining the carrier 372. Gripping effectors 378
having non-vertical faces are provided to grip sidewalls 370 of the
ingot 364, wherein the gripping effectors 378 are arranged at
non-perpendicular angles A1, A2 relative to horizontal actuating
rods 380. Holding the ingot 364 as shown (e.g., proximate to a
bottom portion thereof) using the gripping apparatus 362 leaves the
upper end face 366 and upper portions of the sidewall 370 available
for processing using methods disclosed herein.
[0291] FIG. 43B is a schematic side cross-sectional view of a
second gripping apparatus 362' for holding an ingot 364' having end
faces 366', 368' that are non-perpendicular to a sidewall 370'
thereof, according to one embodiment. The upper end face 366' is
horizontally arranged to receive a laser beam 376, whereas the
lower end face 368' may have a carrier 372' attached thereto, with
the carrier 372' retained by a chuck 374'. Gripping effectors 378'
having non-vertical faces are provided to grip sidewalls 370' of
the ingot 364', wherein the gripping effectors 378' are arranged at
non-perpendicular angles A1, A2 relative to horizontal actuating
rods 380'. Pivotable joints 382' are provided between the actuating
rods 380' and the gripping effectors 378', thereby facilitating
automatic alignment between the gripping effectors 378' and
sidewalls 370' of the ingot 364'.
[0292] In one example, a 150 mm diameter single crystal SiC
substrate (ingot) having a thickness of more than 10 mm is used as
a starting material for production of a SiC wafer having a
thickness of 355 microns. Laser emissions are impinged through a
C-terminated upper face of the SiC substrate to form subsurface
laser damage. A sapphire carrier is bonded to the upper face of the
SiC substrate using a thermoplastic adhesive material disclosed
herein, and thermal-induced fracture is performed to separate an
upper (wafer) portion of SiC from a remainder of the ingot. Both
the Si-terminated face of the separated wafer portion and the
C-terminated face of the ingot remainder are coarse ground using a
2000 grit grind wheel (e.g., a metal, vitreous, or resin bond-type
grinding wheel) to removal all visible laser and fracture damage.
Thereafter, both the Si-terminated face of the separated wafer
portion and the C-terminated face of the ingot remainder are fine
ground (e.g., using a vitreous grinding surface) with a 7000 or
higher grit (e.g., up to 30,000 grit or higher) to yield smoother
surfaces, preferably less than 4 nm average roughness (R.sub.a),
more preferably in a range of 1-2 nm R.sub.a. On the ingot
remainder, a smooth surface is required to avoid any impact on the
subsequent laser processing. The wafer is to be CMP ready and of
sufficient smoothness to minimize required CMP removal amounts,
since CMP is typically a higher cost process. Typical material
removal during fine grind processing may be in a thickness range of
5 to 10 microns to remove all residual subsurface damage from the
coarse grind and any remaining laser damage (both visible and
non-visible to the naked eye). Thereafter, the ingot remainder is
returned to a laser for further processing, and the wafer is edge
ground and subjected to chemical mechanical planarization (CMP) to
be ready for epitaxial growth. Edge grinding may be performed
between coarse and fine surface grinding to avoid any risk of
scratching the fine ground Si face. Material removal during CMP may
be in a thickness range of about 2 microns. Total material consumed
from the substrate (ingot) may be less than 475 microns. Given the
355 micron final wafer thickness, the kerf loss is less than 120
microns.
[0293] Technical benefits that may be obtained by one or more
embodiments of the disclosure may include: reduced crystalline
material kerf losses compared to wire sawing; reduced processing
time and increased throughput of crystalline material wafers and
resulting devices compared to wire sawing; reduced laser processing
time compared to prior laser-based methods; reduced forces required
to effectuate fracture along laser damage regions; reduced need for
post-separation surface smoothing to remove laser damage following
separation; reduced crystalline material bowing and breakage;
and/or increase reproducibility of thin layers separated from a
crystalline material substrate.
[0294] Those skilled in the art will recognize improvements and
modifications to the preferred embodiments of the present
disclosure. All such improvements and modifications are considered
within the scope of the concepts disclosed herein and the claims
that follow.
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